Well, they have a number of issues.
1.) The biggest issue is that they had to build a whole new team to make the game. BioWare Montreal was only like 30-40 people at the end of Mass Effect 3.
2.) Switching engines takes a while, though Dragon Age did a lot of the heavy lifting there. However, it still takes time since they presumably need some features Dragon Age didn't have.
3.) Dragon Age is eating up a lot of staff. DICE mentioned that there was a BioWare project that had over 400 contributors a couple of years ago, so the bulk staff that would move on to put Mass Effect into full production was busy on Dragon Age until very recently.
4.) I get the implication that this is supposed to be a much larger game than previous Mass Effect titles, and is also changing a lot of mechanics, so they need time to both work out how the game will function and then make a ton of content for it based on what they decided.
Overall EA seems likely to be aiming for three year cycles out of BioWare rotating between their three retail products.