[Overread]

Because any new alien encounters would simply be repeats of encounter they have done before. The covered so much material that it would be very hard to give the series any originaillity - something that so far only DS9 has done by being a saitionary setting and by involving a more political storyline than exploration.

Also I feel that any sci-fi series has its own level of technology, go above that and you lose the feel of the show - - travelling to another galaxy, bye bye warp drive, or call i t hype drive - It just loses its origianl feel and won't be star trek any more but something else with some ties - which will hold it back.
Rather make something new and give it its own life and setting