Why is there no universal standard procedure for military intervention when it comes to genocide ?
Actually, there is. The United Nations has a specific international law for Genocide called the "UN Convention on the Prevention and Punishment of Genocide". This international law binds all signatories and requires them to intervene in genocide incidents... However, what counts as genocide is... fuzzy.
The law states to effect that genocide is:
actions, committed with demonstrable intent, to kill off a national, ethnical, racial or religious group.
It should be pointed out that certain categories like sexuality, disabled, or political doctrine are not list under the definition of genocide and if you're wondering why, keep in mind that this was passed in 1948, during a period of time where the USSR was firmly under Stalin, who was actually systemically mass killing people for more political reasons than anything else.
The other key issue is that in order for a "legal" genocide to have occure, it must be demonstrated that it was co-ordinated. The Nazi's Genocide of Jews (along with other minority categories) was discussed at length by top ranking government officials, but some cases of genocide aren't so cut and dry... Where it could be argued that the policy was to take "genocide-like" actions, it can be hard to say that it is authorized by a government policy or an unfortunate side effect of some other policy that was just implemented poorly rather than an actual desire for the extermination of a group of people that meet the above.
Digging deeper, there are other reasons for non-intervention... For one, any thing of this nature will have to pass through the UN Security Council. The Permanent Members (United States of America, United Kingdom, France, USSR/Russia Federation, Republic of China/Peoples Republic of China) all have veto powers so any resolution must meet all five of those member's approval... this rarely happens. The USSR/Russia Federation is probably the worst user of the Veto, with one early ambassador gaining the title of "Mr. Veto" but the United States is no slouch either. Also interesting to point out that all of those nations have rather colorful parts of their past that they don't like to talk about. This was also during the cold war and Russia, China, and the U.S. did not play well together... the UK and France tended to lean towards the United States (and just so we're clear on China, they rarely vetoed on any matter that had no affect on their interests but they did not politically like, favoring abstaining from the vote). Getting all five of those nations to agree on Pizza Toppings would be worthy of a Nobel Peace Prize, let alone that they needed to stop a genocide.
As for the two genocides you listed, well there are also some other factors that contributed. In the case of Rwanda, the U.S. (along with much of the UN) did not intervene because the disastrous Battle of Mogadishu (the battle that gave us Black Hawk Down) was still fresh in the minds of all involved in the Somali Intervention. France (and Belgium) hindered efforts by evacuating only French Expatriots and refused to save any Tutsi who boarded their trucks, including forcing them to get out at Government Check Points where they were killed on the spot. Then, when France returned to lead in the creation a UN sanctioned safe zone, many of the Tutsi saw this as a Safe zone for the Hutu, who were now in danger from the Tutsi backed Rwanda Patriot Front forces (RWP) that were now fighting back against the genocide.
As for the Rohingya Persecution, I think that at this moment the events are too recent to reliably speak with the 20/20 hindsight I discussed with other issues. The true extent is difficult to ascertain given the government highly controlling media access to the state where it is occurring.
In summation, there are a lot of legal assesements and political ties that often get in the way of doing what's right. I generally think that on these areas, the UN is woefully incapable of handling it's responsibility to stop these atrocities but each case has it's own individual reason for lack of response. Keep in mind, that this isn't unheard of. The crime is realtively new and even successful prosecutions take decades. Not every genocidal action is predated by a political manifesto that spells out what the leadership intends to do to minorities if they get power... and even then, Hitler's actions were so outlandish that many people were convinced the victims were making it up. Many of the troops who first went into the camps were briefed on what they were likely to find... and even they weren't properly prepared for the true extent of what they found.