Because it costs a lot more than a weekly anime series. Movie quality animation comes with movie level costs.
That wasn't even movie quality; that was Ultra Instinct Goku vs Jiren quality, which is about 100,000,000 times better quality than what Heroes has been pumping out with its one episode per month.`
Yeah, it's not like the recent Broly movie had some incredibly-animated battle scenes that weren't CGI. Nope, not at all.