Well, most dentists would recommend most toothpaste. They're a lot more concerned with making sure people are brushing regularly than they are with worrying about the brand of toothpaste they use.

That kinda means the exceptions are weird and highly dependent on the dentist and toothpaste brand involved. Maybe they think it's overpriced, maybe they think it's less effective, maybe they have ethical concerns about the parent corporation (looking at you, Crest), maybe they're super picky about what they recommend, maybe they've never heard of the brand before, maybe they just got lousy swag from the company rep and they're being petty. It could be all kinds of things, and the ads certainly won't tell you what they are.

I asked a dentist once. Their opinion was it was most likely the dentist not endorsing because toothpaste isn't really needed, just the scrubbing action is needed more!