That's a really silly comparison.
B89 came out after comic books in general went very dark for their time with the Bronze Age of Comic Books. It also came out in the 80s a few years after Year One, The Dark Knight Returns, and again just darker Batman comic stories.
And going along with that: Batman started dark, went lighter, then went dark again. TMNT started dark, went light and has stayed relatively light in most mainstream incarnations since 1987. In fact they were only dark and gritty for a few years. A vast majority of the the time of the Turtles being mainstream popular wasn't the dark and gritty stuff. So Batman going to 89 levels wasn't as out of the ordinary compared the TMNT which is way more known for being lighter fare. Taking a franchise that has largely been kid friendly for a majority of its popularity is not like Batman who has bounced back and forth in tone during the character popularity in the 1930s-1989
And the "just put them on a streaming service" is just silly too. Whether it's animated or live action that's gonna cost something. Something to make and something to market. Why waste the money, even if it's low budget, on something that most people aren't going to care about?
And saying something along the lines of "We would've stayed with Silver Age Batman on screen with that thinking" doesn't make sense either. Because as I said above, Batman was in a dark period when 89 was being made anyway
There's no benefit of them going back to that aesthetic. Just like there's no benefit for Batman go back to the 1930s incarnation
And besides it's owned by Nick/Viacom. They're not gonna do it. But once again, they'd be stupid to