As we all know the fan reaction to Mass Effect 3's ending was mainly negative (with a small percentage liking it, me included; we can discuss that later.), as it didn't provide any really closer to the choices we've made through the entire trilogy.
Anyway I was looking through some old trailers and I saw that it got delayed to March, then I remembered the script got leaked around November (I think). Then something clicked in my mind "What if they ran out of time?". You may be thinking "What the hell are you talk about?", well I shall tell you.
As you know there was a script leak, meaning that Bioware's writers had to change elements in the story of Mass Effect 3, and to change a story requires a lot of time. As we know EA is not exactly the nicest game developer in the world, and with Mass Effect 3 already delayed. EA was probably telling them that they didn't want to delay Mass Effect 3 any longer, even-though they were not happy with the ending, then they decided that to release the game with the ending it has now and work on the DLC to improve the ending, as this provided them more time to work on the ending they wanted.
This is just a thought that came to me that tries to explain the ending that many fans have complained about. Feel free to this disagree with this it's just a thought that came to me trying to explain way the ending for many was disappointing.