Well, I was around when the 5200 came out, I had an extensive 2600 collection and yet I bought a ColecoVision instead of the 5200. Know what one of the reasons was? The crappy controllers on the 5200 not self centering. And I'm sure I wasn't the only one who didn't like them. So I sold my 2600 lot and started playing Donkey Kong on an almost as crappy controller (but at least it self-centered!).
Again, that's just my opinion, but who is to say it's not the reason the 5200 didn't sell well? I also think the fact that Atari didn't advertise a 2600 adapter for the 5200 (at least not right away, not from what I remember) and Coleco did was a factor as well. How many kids had more than one game system back then, or more importantly, what parent was going to buy their kids another one? (OK, mine did, but I was special). Coleco marketed their console better in my opinion, and I believe it outsold the 5200, did it not?
So you could say that the reason it did not sell as well or was not embraced as much as the other second generation systems was due to a number of factors, one of which may have been what was perceived as a poor design of the controllers with their lack of self centering, blah blah blah... (don't quote me on the blah's)