Because the winds are blowing in his favour. Xi (and the rest of the world) have watched Russia take land with little repercussion. The reaction to the invasion of Crimea was mild and allowed for the full on invasion years later. The rules have changed and now leaders are seeing that taking land, which used to be seen as a relic of the past, is back on the menu.
The US and a lot of the west is afraid to get involved and do what needs to be done to stop Putin, and Xi knows that if he waits for the perfect moment Taiwan will be his.
The US is building domestic chip manufacturing in case they lose TSMC, and once it’s operational one of the main reasons they consider Taiwan an asset will be gone. Xi likely is hoping the war in Ukraine and Israel will continue and “news fatigue” will render people uninterested in a new conflict. I’m not sure what the final nail will be that makes him move, but a controllable president like Trump getting in is a good bet.