It's also probably has a lot to do with the programming behind the site. If we start having "forum likes", "story likes", "blog likes", will the coding that the mod queue relies on, be able to cope with having multiple formulae in order to determine when an author no longer needs their stories modded prior to posting? And as we know, when computer coding becomes more intricate, it just means that there is more that can go wrong.
I have always seen the whole "rep" thing as in indication of how engaged a particular individual is with the site as a whole. So those who post multiple blogs, regularly post in multiple forums and post multiple stories are more likely to have a higher "rep", than those who post say one story (no matter how good that story is).
Anyway, we can all see how many forum posts we've made - so that kind of works a bit like a forum rep.
Just my tuppence worth.