It used to be said that one of the keys to Johnny Carson’s appeal was that people wanted to bring him into their home. They liked the guy. When you’re getting ready for bed and want to relax for a while, you just wanted to hang out with Johnny for a while. Even when he wasn’t funny he was engaging. Maybe even especially then.
Leno’s got some of that. That’s why he’s popular. It’s not about cutting edge humor, it’s about taking the load off someone before they go to sleep.