In case it wasn’t obvious, light SPOILERS.
It’s choice. Or really, it’s a lack of choice. I rambled about this in a previous post, but while I posed symptoms of the problem and some knee-jerk not-ideal possible fixes, I skirted the real issue.
It’s not only bad that Shepard had really only one ending; it’s bad that he didn’t really have a choice in the matter. The three different “choices” resulted in the same outcomes: galactic civ ended without the relays; no happy ending for the Shep; his crew fell through a plot hole and landed on some Hawaii planet; it was all a story anyway. Where was the choice?
Why couldn’t Shepard talk the super bad guy into throwing in the towel and leaving them alone? Not believable? Make it believable. Frankly his entire shtick (esp. as the paragon) was talking his way out of things, convincing others to his way of thinking. The fundamental mechanic in your entire game series is the radial dialog; there really should’ve been a way for it to lead to victory. (See Planescape: Torment).
The ending you gave us would be fine, were it not the only ending. By my reckoning you owe us at least one or two more.
You already have my money, I guess. What will you do with it?