Sing and you die
"Presumably they’ve been worried about being upstaged..."
No, some of them are hyper-partisan Democrats who have decided that Trump won't ever be their president, and the rest are those who would be happy to put politics aside at this time, but are instead having to drop out so they don't appear on liberal blacklists and lose their income.
It's SOP for the Left; Attack not just your enemy but also anyone who won't attack with you. Jennifer Holliday (who is black and a patriot) had to drop out because of the death threats she received from supposedly tolerant progressives and other black people.
Basically many of those behind this boycott are filled with unreasoning rage because their candidate lost. Well some are, and others are still hiding in their safe spaces from the Trump-monster...