Because T.V today, would make it so..Rubbish. Roddenberry would just hate it. have you not seen the film? I think thats all your going to see for now. Either that or their a bit stuck on idea's. There has already been 4 different versions. Its hard to not make them all the same :)
It's also because Star Trek: Enterprise's ratings were bad. It was the first Star Trek series since the Original Series to be cancelled by the network. I heard somewhere that there was to be a new series if the 2009 movie was well received. So maybe we'll see a new Trek series soon.