Why now?
Mostly because Israel's current prime minister, Benjamin Netanyahu, is fighting for his political life. He's been under investigation for corruption since 2016, was indicted on relevant charges in 2019, and formally charged in 2020; his trial is underway as of the time of writing (May 2021). Should Netanyahu be found guilty, his political career - which has spanned over 30 years and five prime minister positions - would be over.
Netanyahu, the leader of the right-wing Likud party, has been fighting to have the allegations and later the charges against him dismissed since 2016, which is coincidentally the year that Donald Trump became President of the USA. While the previous President, Barack Obama, was a Democrat and therefore not wholly supportive of Israel's policies towards the Arab world, Trump as a Republican was almost the best ally that Netanyahu could ask for. With Trump's backing, Netanyahu was emboldened to vastly expand settlement activity (see e.g. Trump's order recognizing the Golan Heights as part of Israel), which naturally improved his profile amongst Likud voters while distracting from the allegations against him.
In 2020, with the possibility of Trump being replaced by a Democratic President less sympathetic to Israel's expansionistic policies, a prominent Iranian nuclear scientist Mohsen Fakhrizadeh-Mahabadi was assassinated, almost certainly by Israel. Again, this act deflected focus from Netanyahu's corruption case while increasing tensions in the Middle East; had Iran retaliated, there is a good chance that open war would have ensued between Israel and Iran, which would have guaranteed Netanyahu's position for its duration.
On 6 May 2021, Palestinians began protesting against an expected court order that would evict Palestinians from Sheikh Jarrah in East Jerusalem (an area of land captured by Israel from Jordan during the 1967 Six-Day War, and that had no Jewish families since 2000). This escalated into Palestinian protesters throwing stones from the grounds of the Al-Aqsa mosque (the third-holiest site in Islam, currently also under Israeli control). Israel responded by sending riot police into Al-Aqsa, firing tear gas, rubber bullets and stun grenades.
This inflammation of tensions was increased after Hamas issued an ultimatum to Israel on May 10, requiring the withdrawal of all Israeli security forces from the Temple Mount (where Al-Aqsa is located) and Sheikh Jarrah by 22:00 of that day. If this was not done, Hamas threatened that it and the other paramilitary groups in the Gaza Strip would resume attacks against Israel.
Since it's very unlikely that Israel would comply with this demand, the rationale for making it initially appears unclear. However, Mahmoud Abbas, the governing leader of Gaza/Palestine since 2009, recently indefinitely postponed long-overdue elections that were widely expected to weaken his Fatah party while strengthening their rivals Hamas; this effectively shuts Hamas out of political power, and presumably as a result they chose to remind all parties in the Israel-Palestine conflict of their power by making a demand of Israel that would naturally be rejected, thus giving Hamas justification for attacking.
As Israel did indeed ignore Hamas's directive, the latter resumed rocket attacks against the former, which has responded harshly with air strikes on the Gaza Strip, including on the headquarters of news organizations. This hardline response is consistent with Netanyahu's continuing attempts to deflect attention from his trial.
tl;dr
It's because both Israeli and Palestinian leaders are playing politics. On Israel's side, Netanyahu is using any and every thing at his disposal - including but not limited to military force - to deflect attention from his trial. In Palestine, Abbas's postponement of an election that would potentially see him unseated from power - which would lead to scrutiny of his personal finances, which he has been rumoured to be increasing using funds intended for Palestinian residents - has prompted Hamas to assert its power.