Well, because it's...not really? I mean, I guess there's nothing hugely bad about it per say, but the shooting is shallow and basic, enemies have no real variety, the melee is wonky and mostly useless, the platforming is brain-dead, and the puzzles are mixed ranging from fun to similarly brain-dead.
It's enjoyable to satisfy going from point A to B, but if you stripped away the technical wonder and storytelling, and had any of the games stand purely on its gameplay alone, do you think it'd get the same reaction it has? Some may try to say it would, but I highly doubt it would at all. The game's strength's have always leaned on its strong storytelling (especially considering the poor quality of most game's storytelling) and its amazing technical boundary pushing.
As far as my rank goes:
Among Theives
A Thief's End (so far, not finished yet)
Drake's Deception
Drake's Fortune
I haven't played Golden Abyss, but I have a hard time believing it's not at least as good as the first one, though.