Trek and Wars have more to distinguish them than most cop shows. Yet no one seems to question the need to flood the airwaves with an uncountable number of interchangeable series of that genre.
Cop shows are cheaper and there's a larger (and apparently far less critical) audience for them compared with space opera.
To answer the OP, "it's utopian versus dystopian" is basically the right answer. Every other space opera I can think of is dystopian (
Farscape, nuBSG, Firefly), cynical (
Futurama) or just brainless (
Stargate). I can't quite decide if
B5 is cynical; it might be cynical-utopian. But you definitely would never get it mixed up with
Star Trek and if JMS ever attempted a
Star Trek series, it wouldn't work.
As for
Star Wars, the jury is out on that one since
The Clone Wars is busy rewriting the book, especially on the metaphysics. It might be utopian in a more metaphysical/elitist way than
Star Trek (the cosmos can be perfected, but only by some kind of Chosen One) or it might just be babble.
Come to think of it,
B5 might be utopian-elitist. How else could Sheriden end a war just by yelling at the combatants?
If CBS was smart, and of course, they are, they would beat Trek to the punch of putting an optomistic space opera on tv.
Being smart, CBS will of course do nothing of the sort. They're not in the business of deliberately losing money. The only realistic places for space opera are either SyFy in the miraculous event that they get their shit together, or elsewhere on cable.