Comedians need at least two things to be successful, the ability to convey information with a large number of emotions and a constant attention to how it is received. The most well known can switch between emotional and cold or happy and angry while playing two people in conversation. Both are incredibly important skills for acting in roles!

I think it is more that comedians who have already honed those skills on the stage make great actors as long as they don’t carry ego or something over, just like established stage actors moving over to movies and TV. They have experience that carries over.