Well let's be honest, it's because he's Jewish.
Most of the people withholding their votes due to Gaza were Bernie Sanders supporters.
We obviously have an ontological disagree over Zionism and its relationship to Judaism. But clearly, the sort of voters who are at risk of defecting from the Democratic coalition see a hard distinction between Jewish politicians and Zionist politicians.
And yes, Bernie is a mild Zionist in that he believes that Israel has a right to exist as a political entity but he’s very critical of the tiered citizenship, the killing of civilians, the checkpoints, and seems willing to withhold military aid from Israel if he were president right now.
Shapiro and most US politicians are hard Zionists in that they may do a little hand ringing but they are mostly fine with the status quo but in the region and back here in the US with regard to anti BDS laws.
If you put a hard Zionist gentile like Ritchie Torres in the VP spot they would have similar issues with certain parts of the left flank of the party.
Antisemitism in the US is largely an outgrowth of anti Blackness/white supremacy. American anti semites hate the Jews who they see as promoting desegregation, civil rights, Black music, and helping refugees get into the country.
US based anti semites generally support Israel because it removes Jews from the US, and will hasten the Rapture. But if you intend to stick around and be a woke professor, activist, lawyer, politician, or entertainment figure, the antisemites of the US, who are almost entirely housed on the Right, will hate you.
The left hates Zionism, the right hates Jewish social Justice allies, and the number of American who, per se, hate all Jews is vanishing small.