The US might pressure him to assemble a government of national unity on pain of once again suspending military and intelligence aid if he refuses to dilute his power in lieu of holding elections...
Ukraine extended martial law until 6 August following Zelenskys request earlier this week, which will prevent elections from being held over the summer like The Economist claimed late last month was a scenario that he was considering in an attempt to give himself an edge over his rivals. This move therefore exposes his fear of losing re-election. Its not just that hes very unpopular, but he likely also fears that the US wants to replace him after his infamous fight in the White House.