Firstly because Ray Fisher's experience with WB means they've pretty much buried any chance of seeing the character on film again but also I'm of the opinion that not every character can carry an entire movie.

What would a Cyborg movie be once you've told his origin story? It'd just be CGI punch ups with second tier bad guys. There's not much to say about Cyborg other than his origin story, it's why his solo comics don't have much going on. As far as I know he hasn't had a solo comic since 2018 and even then he's not had that many successful runs previous to that so it's not like theres a treasure trove of stories to go digging into.

Giving every character you have a movie means you end up with movies like Antman and The Wasp. Forgettable trash.

I'd rather have this 30 minutes of killer Cyborg story than a 2 hour slog cobbled together from non existing ideas.

Sometimes less is more.