Personally, I think the writers/producers wanted to keep things accesible for non-fans. Which is absurd really, when you try and make a movie based on a tv-show, unless you do a complete reboot.
So, they decided on a new villian, instead of one from the show. I wonder how non-fans would react to Wrath Of Khan, having never seen the original series' episode it was inspired by.
Sure, it's easy for fans to say that the backstory was explained well enough for non-fans, but we know the backstory.
So yeah, I think it was about keeping things simple for non-fans, casual viewers.