AVP '99 had a few points in its favour. First of all, its use of multiple perspectives and different play styles (particularly wall-walking) were new and innovative. Also, it was one of the first high-profile games to require a dedicated GPU, so it was something of a benchmark game for tech-heads, which FPS games have traditionally attracted (think Crysis).
AVP '10 obviously had the innovative gameplay angle taken away from the get-go, being the third entry in the series, let alone all the imitators in the intervening years. Also, while it's one of the relatively few games to make full use of DX11, this wasn't as big a draw on the tech-head scene (maybe because those without DX capable systems weren't aware of the difference it actually made). It may be fair to say that the requirement of console versions may have held it back in this regard, but that's debatable (equally it could be said to have greatly increased its market).
Bottom line though, AVP '10 wasn't a bomb. It topped sales charts in many countries for a number of weeks, and received generally fair to positive reviews. People just get lost in the negative hyperbole surrounding these things. At most you could probably chastise it for being unambitious, but by no means was it a bad game.
