While the AC chargers may have a longer life or be better (depends on who you ask I suppose), not everyone has access to an electric outlet anywhere close to their fenceline....and not everyone wants to have a car battery sitting so close to their pasture.
I've got solar chargers and they work great. Other than the one unit I bought a few years ago that was defective, they've all had very long lives. The one I've got on the fence for my drafts is about 5 years old and it's never missed a lick. Plenty of bang to keep my fence-crashers in.