Why the ending of Mass Effect 3 sucked in two words (the shortest length I could come up with): Screwed narrative.
That, in itself, means nothing though. What was screwed about it? Is that fully indicative of it? I watched all of the smudboy deconstructions on YouTube and they detail all of the reasons why it falls apart. Even if you think about the more obvious points brought up instead of the particulars, there's still a crap-load to be drawn to.
Really, BioWare's writing is bad and they should feel bad.