1: they are presently trying to come up with a reason for a 5 gb patch that doesnt tell everyone that so much was buggered they need to rerelease the whole game with decent textures and coding optimising.
but fear not...they will fix it,as they always do....just takes a year or 2 for them to do it,they have a lot of investors to sort out first.
2: or is it because the steam mod golden pot of user created thievery isnt working yet ?
they have to find a way to justify selling you mods created by users that used to be free,they know we are pretty dumb,but this ones going to take some twiddling to sneak it past the usual user

3: so much botching they cant get it done this fast ?
4: they have the sales figures,why bother ?
5: plain olde fashioned incompetance ?
so many reasons possible,and being as they are leaving us to guess,then lets go crazy and see who wins the lottery