My guess is that the story of the game was too complex, and media executives tend to dislike complexity. Dumbing things down in favor of the lower denominator is not uncommon within the media branches.
Securing the possibility sequel might also sit well with shareholders.
A technical, or game design aspect though, is that the story needs to make sense for everyone, including people who didn't bother to follow the story or who went straight for the ending. This ending does secure that demand, which may have prompted the move to use it instead of a more complex, and thereby too difficult ending for 'most players'.
That is an assumption and there is no way around the fact that it trivializes the invested players experience. But it does make the choose a bit less "let's troll them".
Time and resource restrains are also factors, as the game was delayed for a reason, Deception was an unedited POS, and the former main writer(s) left Bioware.
So I imagine not everyone was too pleased with it, but there are only so many possible designs that solve a range of problems. (the narrative problem is one, but also gameplay, difficulty, and so on).
Still: any attempt to understand why the endings are there will assume rationality of some kind. It is however wise to assume that this is not true, and that the actual reasons will remain unknown until Bioware employees start describing the design process in detail.