This is the college down the road from me. They used to win NCAA Hockey championships all the time back in the 60s and 70.

-Crappy (or non-existent) marketing.
-Poor timing (released alongside other titles).
-Only on certain platforms.
-Catering to a niche audience.

There's probably more, but that's a start.

The reviewers were paid off. Like SimShitty for example. It had positive reviews near release and only changed after most players started voicing their displeasure.

I'd add a lack of identity, games that are neither recognizable nor memorable, at least until you actually play them. I think a good and rather recent example would be Remember Me.

Then there's a lack of USPs (unique selling points) or USPs that are unmarketable (a bit related to the "identity" thing above). You couldn't quite summarize System Shock 2's or Deus Ex' unique gameplay in a few words and even with extensive descriptions people weren't quite able to imagine it. Heck, even after playing Deus Ex for twenty minutes at a friend's place back in the day it just appeared like a rather mediocre shooter to me, I had to play it for several hours at my own place to get hooked. It's no wonder that it took those games a while to develop a following.

Also some games appear to target another audience than the one they are meant for. Think of Beyond Good & Evil or Psychonauts, for example. Both appeared to be meant specifically for kids and were indistinguishable from all those crappy movie tie-ins on the outside. Only several years later gamers got accustomed to the thought that cartoony graphics don't mean childish gameplay. Even when Team Fortress 2 came along the cartoony style was still a rather bold move.