Because it's a no-win political issue, and Harris isn't the type of leader to do the right thing just because it's the right thing. Anything she says will piss somebody off, and she still thinks she could run for office again in the future.
Ironically, if she were the type of leader to actually lead on this issue, it might be worth supporting her in the future. But she won't, so it isn't. She's yesterday's leftovers, and the Democrats need to turn over the old guard.