I've been hearing a whole lot of radio sponsorships as of late, too. They seem to be focusing on getting their name out there, which is what they need to do.
I definitely miss John Mahoney, but I suppose they're going more for a clean-break identity rather than the continuity of using the familiar voice.
Might also be a matter of cost versus benefit. Midwest built a good deal of their advertising around the celebrity voice talent of John Mahoney (although a good number of commercials used a generic female voice talent, too) and they almost exclusively used that voice in MKE, MCI and OMA media markets. After awhile when you heard the voice on a commercial, you knew it was the voice of Midwest. Unless Frontier was going to do a similar effort to make his the recognizable voice of Frontier, paying a premium for him likely didn't pay. If he were to just be the minor role of badger voice, it would probably be missed by plenty of people in the legacy Midwest media markets and totally wasted anywhere else the Frontier commercials ran.
I liked the idea of keeping John Mahoney for continuity, but I suppose it makes sense why it's not happened. Although I'm a long-time Frasier watcher, it was a long time before I first realized the guy who was doing the Midwest commercials was John Mahoney. So while it's a classy trademark to us, I'm not sure it means as much to the average person.