Because it's not politically expedient for him. Labour could lose a significant number of votes from Muslims over what's happening in Gaza, but are unlikely to lose many if they just wait for the news cycle to move on and the attention to go away, which no doubt will happen shortly.
Maybe Starmer does really care about antisemitism, maybe it was just to differentiate himself from Corbyn and give the appearance of being a strong politician unafraid of making "difficult" decisions.