Maybe the biggest reason he won’t speak out is because he won’t get reelected. Which is scary in itself. But, let’s not pretend Pence is some sort of Saint. He is evil too.
After not having crimed Trump into a second Term, he has to know that the MAGA crowd would rather hang him than let him have any office while they’re in power. So if getting elected were his primary concern, making sure Trump is not put into power would be Pence’s primary objective. And he’s just laying low, not working against Trump, so we can be sure that getting reelected is either not his goal or he’s more afraid than he is ambitious.