Time and money constraints. Moviemaking can never bring the same imagination as books or animation or video games can. That's why it's hard having a good game to movie adaptation. Probably one of the best so far is Tomb Raider, mainly because Lara Croft and the whole game involved humans, so finding a hot woman with a nice body could easily suit Lara Croft, because that's what she looks like in the game.
Yeah, I expected more of Doom. It was...excting at times, but I really wanted more than Special Op guys in the normal battle dress with modified H&K G36's. One thing that puzzled me and made me laugh the most was the lack of technology in the movie. Sure, the BFG and the guns we're cool, but where were the Sentry Bots? Only a flashlight? It's that far in the future and they don't have nightvision/thermogoggles? It's just weird...