Kotaku's lengthey report on what went wrong with Mass Effect: Andromeda reveals essentially that Biwoare's A-Team all began working on Anthem after the March 2012 release of Mass Effect 3 while the B-Team was left to work on Andromeda. What that means is that instead of developing Andromeda at BioWare's main HQ in Edmonton, it was primarily constructed at the company's Montreal facility which was initially created in 2009 to develop DLC for Mass Effect.
The A-Team back in Edmonton, and under the leadership of executive producer Casey Hudson, began work on a project that was codenamed, Dylan - because as Hudson put it, "BioWare wanted to make the Bob Dylan of video games, a game that would be referenced for years to come." Of course, at E3 in June, it was revealed that Dylan would be officially released as Anthem. Hudson stayed on Mass Effect: Andromeda' s development during its early stages but soon left the offshoot studio to its own devices as he focused on Anthem.
Initially, there were some in the Montreal office who wanted to produce a Mass Effect prequel set during the Contact Wars- humanity's first contact with aliens. However fan and focus group feedback resoundingly balked at the notion of a prequel, preferring a sequel that continued the story of the original trilogy.
Over the course of development, it was decided that Andromeda would be an exploration game set in a different galaxy, free from the story entanglements of Commander Shepherd's journey and the fight against The Reapers. To take the exploration theme to the next level, it was coincidentally determined that the game engine would use algorithm-generated random planets to increase the feel of uniqueness and increase repeat gameplay. Of course, as No Man's Sky can attest, the technology just isn't quite there yet for such a game.
Like Hello Games, BioWare couldn't make the algorithm for randomly generated planets work the way they wanted but where they differed is that BioWare abandoned that concept 18 months out from launch in favor of a more traditional action-adventure mold. Another factor according to unnamed sources, was that the Andromeda team felt as if they were understaffed for key things such as dialogue and facial animation. Of course, it stands to reason that the bulk of Bioware's manpower was focused on the new IP, Anthem.
Whether that's the actual case or not, the Edmonton team did come aboard the project during those last 18 months but that just wasn't enough time to bring the game up to the developer's usual standards of excellence. In the end, the Montreal staff was "scaled down" an industry term for layoffs and the remaining team members were split up to work on Anthem and the next Dragon Age game.