They're probably waiting as late as possible so they know what the shot is. Do they go minority or minority-woman when the minority vote is already in the bag? Or do they choose a more conservative white guy or whitish female like Tulsi to try and eat away at Trump's white populist support? With corona and race-riots a lot is unclear. So, they're waiting

It almost doesn't matter either since the election is theirs to lose. Trump would lose to a corpse at this rate - and that's practically what Biden is