I wonder if poor DLC sales could have been a factor, there's been a mindset over the last few years that a game is a platform for generating more cash with supplementary content. A:I's DLC has been badly received, so I assume it didn't sell well - I know I didn't buy any.
The game itself sold as well as you could reasonably expect a series-starter to manage. Yeah, there have been Aliens games before, but it's been 14 years since the last well-reviewed one, and that wasn't ported to any consoles. A:I wasn't just effectively starting fresh, it was starting with a poisoned chalice, as Colonial Marines was famously terrible and AvP3 had a middling reception. 2.1m+ sales was good, under the circumstances, and could have been improved on with a decent sequel, after all the goodwill this game's generated.