god dont mention the season 8 finale ;(

The heroes were never introduced or even referenced in the film, so leaving them out doesn't cause any problems. This is a stand-alone film entirely in its own continuity; for all we know, the League haven't formed yet, Superman hasn't even met Batman yet, and obviously no Supergirl around. The film is what it is, a loose adaptation of the Death of Superman arc, a looser adaptation of the Funeral for a Friend arc, and an original story for the final act that're all combined in an entertaining stand-alone feature. If you'd like to see a Doomsday arc with the league, you can always check the episodes featuring him in Justice League/Justice League Unlimited or Smallville's Season 8 finale.
Any Buffyverse comic that's post-"Not Fade Away" is non-canon, my PoV says.

it doesnt matter its logic and coomon sense if you introduce a group of superheros then have a deadly threat you need a good reason for them not to get involved

this isnt about the JLA its only supposed to be about superman. i dont see why everyone seems to think this is supposed to be a direct port/adaptation of the comics. its a perfectly fine standalone piece.
just like the superman live action movies have nothing to do with the comics.

This isn't the comics.
Nevermore