The reason is simple, there is no game like it out there. There are no other scifi rpg out there. Nothing that combines storytelling with galaxy spanning adventure and action like ME games.
I just hope they make MEA better than ME3 I kinda want more games like this.
I guess I might be living proof for this thought. I still HATE the Ending of ME3 and think Bioware made it all wrong from the very first minutes of ME2 up to Star Child, completely ruining the potential of the story, or any story (ME2 was just ... a long recruiting mission without much connection to the overall plot, which led to ME3 feeling way too rushed for the giant conflict it was made up to portray ... but enough of thi.
Andromeda? I am still unsure what to make of it, but hopeful. Because really, there are no games like it, with a potentially female bad-ass protagonist in space in an RPG. Its all fantasy or pure FPS at best...so the sheer need drives me back into this universe ^^
Also, Bioware is clever enough wit the setting in a new galaxy, I'll give them that. They can totally ignore all the bullshite they created with the last two MEs, take any elements that were good (for nostalgic fans like me) and don't have to worry about alienating potential new customers (the main reason as I see it why ME'S story failed - the fear of really dedicating to tell a comprehensive story where you NEED to have played other titles to "get it" - they tried a mishmash of this but ME2's story for that reasons fails, and the trilogy with it...)
And if they are REALLY clever, they will never try to go back to the whole "all your choices matter" and connected trilogy-thing. Will never work anyway. They can keep the protagonists maybe, but each ME-game of the future should be mostly stand-alone, and the overall-plot should be linear, with only minor details varying from player's choice to player's choice - that don't pretend to affect the big world....