Mainly because it's not cool and edgy.
I'm a Transformers fan first and foremost. Not only have I gone through loads of godawful iterations, but my fandom's been shat upon for decades. So now Thundercats Roar comes along and you know what? I'm able to accept/tolerate different takes on beloved childhood IPs.
I liked Roar. There are a couple misses but on the whole, the show hits the spot.
I love the Berbils and their creepy Masochism. Driller existential crisis in his episode was nice. Tigra x Mumm-Ra are great as besties. The Snarf's day off episode was great too. And of course, the finale with the Cats Lair.
Lion-O's eh for me, he appeals more for the kiddies I guess. Cheetara's just mostly there, although her focus episode was another banger. I'm generally eh on Tigra's buttmonkey status as well.
But yeah, I mean, I realize this thread is bait but I actually did like Roar. Despite the Teen Titans Go route it takes, it's more respectful of the source material than TEE-LA: MASTERS OF THE UNIVERSE: REVELATION!