Why hasn’t Ukraine joined the North Atlantic Treaty Organization (NATO)?

Question

Ukraine was part of the USSR and made themselves independent. Vladimir Putin called the separation of the USSR a "major geopolitical disaster of the century". I am aware of the Russian annexation of Crimea (a part of Ukrainian sovereign territory) and Ukraine needed or had the will to join NATO. My question is, why hasn't the Ukrainian state joined the North Atlantic Treaty Organisation? From a Wikipedia article, I found out that 69% of the Ukrainian people support an agreement with NATO. So what's missing?

Recently the US and NATO are doing joint military exercises near the Crimean Peninsula, where the Russian Federation, Chinese, Iranian, Belarusian, Armenian, Pakistani and Myanmar Armies are also doing joint forces in Caucasus 2020, and by which tensions are sparking. Belarus and Russia are also militarizing near the Border to Poland (EU-State, NATO-Member) since they share Pan-Slavism. These military activities pose great fears to the US, remaining of the European-Union and NATO.

My point is if Ukrainian and NATO forces do joint forces, couldn't the Both be combined (Ukraine in NATO)? I mean, if Ukraine joins NATO, it wouldn't be beneficial and will even harder for Russia to increase influence and securing Ukrainian Sovereignty as a whole with protection by the most powerful Alliance makes annexation difficult and containing Russian Influence over it's friends borders. The only thing that makes Russia Influence possible is Ukraine not being able to join NATO. Has NATO ever made open statements on Ukraine joining NATO?

• https://www.rfi.fr/en/international/20200921-russia-china-launch-massive-caucasus-2020-military-exercises-belarus-iran-pakistan-ukraine-us-nato | Main Article
• https://www.militarytimes.com/news/your-military/2020/09/20/russian-military-says-us-and-nato-flights-near-crimea-fuel-tensions/

Note: I appreciate every correction (edits) to this question, comments and am also happy if the question is taken down from site when seen as inappropriate.

Answer 1

NATO won't let them in right now.

NATO is an alliance of nations who promise to defend each other's territorial integrity, among other things. There are some catches to that, so the Falklands war did not become a NATO Article 5 situation because those are South Atlantic islands, yet the 911 attack on New York did.

NATO supports the Ukrainian right to make a membership application as a free choice, without a Russian veto. It also takes official note of the application and talks about it, but that is not the same as an invitation to join now.

If NATO were to admit them, it would find itself called to restore the territorial integrity of the Treaty area, and it isn't prepared to go to war with Russia over Ukraine. Generally NATO expects new members to resolve any outstanding territorial issues first -- compare the lengthy process for North Macedonia.

One might call that callous, or a sane refusal to risk WWIII.

Answer 2

One of the many reasons is that as a response to Ukraine's new language and education laws, which took away basically all minority rights from the ethnic and linguistic minorities of Ukraine and banned the usage and teaching of minority languages, Hungary blocks every form of official contact between Ukraine and the NATO.

Answer 3

[W]hy hasn't the Ukrainian state joined the North Atlantic Treaty Organisation?

— because of the combination of internal and external factors.

The external factors are discussed in details in existing answers. Let me focus on internal factors that Ukraine has to improve before it could be awarded with the membership (and the absence of which has prevented Ukraine from membership in the past, which is the direct answer to your question).

• reforms in defense sector that would make Armed Forces of Ukraine closer to NATO standards, which, in turn, is a cornerstone of a country's participation in a defense alliance;
• reforms in security sector and combating enemy's espionage. You literally can't be trusted confidential information of a military alliance if your top government positions are controlled by acting officers of the alliance's main adversary;
  • To give you an impression of how bad things are after the 2019 elections: for more than two years (2019-2021), the Zelenskiy regime has failed to appoint¹ heads of Ukraine-NATO and Ukraine-EU committees. You can't accomplish anything if your profile committee is effectively paralyzed;
• reforms in economy, decentralization, promoting the rule of law, fighting corruption, eliminating the influence of the oligarchy and foreign-sponsored organized crime;

NATO members recognize (see quote below) that throughout the recent years, Ukraine made a substantial progress making these reforms, it's still a consensus that, as of June 2021, the progress is not sufficient to become a NATO member.

Has NATO ever made open statements on Ukraine joining NATO?

— Yes.

The 2008 Bucharest summit of NATO has paved the way for Ukraine's membership to NATO.
The 2021 Brussels summit has confirmed that decision. I'm placing here the article 69 of the joint Communiqué because it not only answers your question in full, but also defines specific steps Ukraine has to take in order to get ready for the membership (referring "internal factors" from the top of my answer). I took my liberty to split the text into paragraphs for readability.

We reiterate the decision made at the 2008 Bucharest Summit that Ukraine will become a member of the Alliance with the Membership Action Plan (MAP) as an integral part of the process; we reaffirm all elements of that decision, as well as subsequent decisions, including that each partner will be judged on its own merits. We stand firm in our support for Ukraine’s right to decide its own future and foreign policy course free from outside interference.
The Annual National Programmes under the NATO-Ukraine Commission (NUC) remain the mechanism by which Ukraine takes forward the reforms pertaining to its aspiration for NATO membership. Ukraine should make full use of all instruments available under the NUC to reach its objective of implementing NATO principles and standards. The success of wide-ranging, sustainable, and irreversible reforms, including combating corruption, promoting an inclusive political process, and decentralisation reform, based on democratic values, respect for human rights, minorities, and the rule of law, will be crucial in laying the groundwork for a prosperous and peaceful Ukraine.
Further reforms in the security sector, including the reform of the Security Services of Ukraine, are particularly important. We welcome significant reforms already made by Ukraine and strongly encourage further progress in line with Ukraine’s international obligations and commitments.
We will continue to provide practical support to reform in the security and defence sector, including through the Comprehensive Assistance Package. We will also continue to support Ukraine’s efforts to strengthen its resilience against hybrid threats, including through intensifying activities under the NATO-Ukraine Platform on Countering Hybrid Warfare.
We welcome the cooperation between NATO and Ukraine with regard to security in the Black Sea region. The Enhanced Opportunities Partner status granted last year provides further impetus to our already ambitious cooperation and will promote greater interoperability, with the option of more joint exercises, training, and enhanced situational awareness.
Military cooperation and capacity building initiatives between Allies and Ukraine, including the Lithuanian-Polish-Ukrainian Brigade, further reinforce this effort. We highly value Ukraine’s significant contributions to Allied operations, the NATO Response Force, and NATO exercises.

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¹) Head of Ukraine Mission to NATO Mr. Georgiy Tolkachov is at the Acting Interim (a.i.) status