[alfiejmulcahy]

Many reasons. Radio 2 seems to mean more as a station than any individual presenter on it. Chris Evans did exactly the same thing as Ken, leaving Radio 2 for Virgin and whilst some listeners left, there was no mass exodus. For many tuning to Radio 2 is a habit no matter who presents on it.

There is also a lot to be said for the ad-free nature of BBC Radio. Moreso than TV, radio advertising is far more intrusive (in my opinion) and puts a lot of people off.

Isn't that why Chris Evans initially on Virgin Radio had no commercials, and was sponsored by Sky, which I think still sponsors his show. I am not sure if that ad free agreement is still in place. 

Graham Norton's Virgin Radio show is sponsored by Waitrose, I wonder if that is ad free too?

Evans' show is now sponsored by Cinch and the programme carries far fewer adverts than the rest of the network, but Evans' now has a couple of advert breaks towards the end of the programme but not many. Norton's I believe is still ad-free, but I only tend to listen to the first hour or two.