This. It's the same reason the Ghostbusters reboot failed.
If you wanna reboot something with a different tone and feeling, that's FINE. There's literally nothing wrong with giving something a new spin, that's what gave us the 2003 TMNT show and the entirety of the DCEU. Spectacular Spider-Man is by far the best Spidey show, and it's got super-simplified designs compared to the 90's series.
Different isn't inherently BAD.
But you can't be surprised when you're met with some skepticism and vitriol when you trot out something that's radically different from what people already know. You're basically asking for them to trust you that what you're making is good.
When you respond to that skepticism with "lol cry more you cis white male I hope this makes you mad sorry sweatie it's the current year and this is cartoons now virgin neckbeard white male fedora", you shouldn't be shocked when people say "okay faggot, fuck you and your show" and the entire thing tanks.
If I were CN, I'd already start planning for what to fill Roar's timeslot with. Unless this ends up being a complete dark horse that takes everyone by storm and subverts every opinion that's been violently lobbed at it so far, I see it getting swept under the rug faster than nuPPG.
Not to mention you can't make toys out of these fucking noodle-arm tube people, which was literally what carried the OG Thundercats show, so they stand to make little to no money off of this thing outside of ad revenue from commercials and whatnot.