...Because it's exactly like Star Wars (Except Star Trek aliens is siller) mixed with a bad adventure show. The only thing that doesn't fit into the definition above is that it isn't an "epic". At least not the series, don't know about the movies as I've only seen the newest one (which, by the way, if you had exchanged the differents terms and sci-fi babble would be exactly like a Star Wars movie.
The guy who did that RedLetterMedia style breakdown of the ME3 ending explained it far better than I will now attempt to, but here goes: Shows like Star Trek are all about the technology, the science part of science fiction. Nearly every problem they encounter is solved by liberal application of technobabble, and the few that aren't are generally solved by diplomacy. Obviously this varies from series to series, but it generally holds true for the entire franchise (except the TNG films which may aswell be an entirely seperate universe for how consistent they are with the show). The shows in general are driven as much by the technology than as by anything else. Things like the warp drive, phasers, replicators, holodecks, etc. They're all fantasy creations but the writers spend a large amount of time educating the viewer on how they work, what their limitations are, how they are powered, etc. Even data's positronic brain is somethign that is constantly explained and referenced to the viewer.
Constrast this with Star Wars, where the technology is just a tool that isn't ever really explained. Nowhere in the movies does anyone talk about what makes C-3PO tick, or how a lightsaber knows when to stop, or what hyperdrive really is. It's all just plot contrivences to move the narrative from point A to point B. The technology in Star Wars is never really the focus of the plot. The plot is more concerned with galactic civil war, the force, character arcs and relationships, good versus evil, etc. Not to say that a space opera can't dwell on technology, or that a "talky techy" show can't dwell on big narrative elements, but generally you'll find they stick to their strengths.
And that's just one aspect of it. In Star Trek, generally if any sort of obstacle rears it's head the characters first reaction is to discuss it. Bones and Spock argue their points and then Kirk weighs in and decides, Picard calls a meeting in the boardroom and the resident experts weigh in, Janeway asks Tuvok and 7of9 for guidance (though I think a lot of the reason people have a problem with Janeway is she often just does whatever she wants regardless of what her command staff says). In Star Wars the characters very rarely stop and take a minute to sit and work out a plan. Obviously they have the breifing scenes before the death star run, but even then it's more of the characters being told what to do by some random guy we've previously in any scene.
Obviously some of these differences stem from the different mediums, Star Trek is a TV show, where generally they have an hour to solve one or two problems, and limited budgets for huge action sequences... Which is why you find Talky techy franchises more at home on TV. ME1 was so original because Talky Techy games are kind of rare, which is why it's so disheartening to see them play fast and loose with things like what the underlying technology is or does.
Also the recent Star Trek film is definatly far removed from the TV series, but then again it's supposed to be. It's a franchise reboot. The whole idea is that they don't want to make Talky Techy slow paced things starring old guys who discuss proton photon conduits and plasma coils. Though I don't think it's really a space opera either... But it's been a while since I've seen it so I can't cite any specific reasons.
I know of at least one exception to this, if Wrex is alive and you attempt to get salarian aid you'll lose all of the Krogan assets Wrex would of given you, but if you destroyed the data, Mordin is alive but Wrex is dead and you got Wreave instead then you can get the (Very marginally weaker) krogan, salarian aid and 30 extra points from Mordin going to work on the catalyst. I know that's at least one example that to game the system and get as much war assets as possible you need an old character dead.
I'm sure I read somewhere that you can get both Wrex and Moradin (including both salarian and krogan forces) if you leave them both alive, saved the council in the first game, and then save the salarian counciler from assassination? Could be wrong though, and I CBA researching it to check.