Because there's probably a good amount of overlap between people who are pro-abortion rights and those who are sympathetic to Palestinian rights. Abortion rights are also an important and immediately critical issue domestically, so interrupting a rally to support those who need access to abortion with a pro-Palestine protest is just going to piss off people at the rally who are already on the side of the protest. It'd be like if a group of pro-choice advocates started a protest at a pro-Palestine rally, it's just bad form when the group they're disrupting probably agrees with them.
I get the protest was targeted at Biden and Harris, but even still, it's a bad idea to protest them at an event that is probably for an issue the protestors also care about. It just signals that there's no reaching the protestors and they're going to be unreasonable, even if Biden comes around to their side and stops shipping arms to Israel.