Is it simply cowardice? There's a bit of that. But they could resign if they were merely afraid.
Is it the desire for power? There's also some of that but what power do you have as a lackey for a madman.
I think it's mostly selfishness. They think there are individual opportunities to make money by remaining visibly loyal to Trump. And so they support him.
It's an interesting voyage around the personality of GOP senators. It begins with dishonesty. Then sails through cowardice, the lust for power and avarice. Navigating the sea of sewage.