It’s not been in Putin’s interest to get rid of him until now. My guess is that Putin is feeling more exposed, or expects the domestic situation to deteriorate, and wants rid of opposition figureheads. If that’s the case then expect other potential rivals to disappear too.
He knows nothing much will happen in response - see all the other opposition who have fallen out of windows - and he's about to get a new 'ally' in Trump so will be handed an almost free reign to do what he wants.
It’s not been in Putin’s interest to get rid of him until now. My guess is that Putin is feeling more exposed, or expects the domestic situation to deteriorate, and wants rid of opposition figureheads. If that’s the case then expect other potential rivals to disappear too.