I honestly don't get why these three are so unpopular. Sure, their Incarnate Formes are bland as heck and blatantly copy-pasted, but their Therian Formes are actually a really cool trio and one of the better Legendary groups in my opinion, from a design standpoint.
Of course, I totally get disliking Landorus for its competitive overuse. But why the whole trio? Tornadus and Thundurus are hardly even used (and when Thundurus IS used, it's generally Incarnate, not Therian, so it's not like that's a mark against the Therian trio), and they honestly have really appealing designs!
Disclaimer: I don't participate that much in competitive formats where Landorus is everywhere - so that hasn't hurt it as much as it does for many people, and even then it's still my least favorite of the three - and also I've never used any of them in serious battling. So this is purely about design; I get that most people's opinions are soured by competitive, and I would have no ground on which to stand if I were to argue that side of it.