Part of it is that enemy AI was massively broken on release. To the point where they would just wander around and ignore you, or just charge straight at you with no regard for stealth. This turned out to be as simple as a typo in some AI settings that completely broke what was supposed to be a game with a tense, "smart" AI that would sneak around and look for ways to attack you when you weren't looking. This one screwup went unnoticed for years, until it was found and fixed by a modder (And, I believe, later patched into the official game).
Broken game on release, broken because there was not the level of care taken with it that would keep them from finding and fixing Tether spelled Teather, where playtesting should have shown that the AI was not working correctly and had them looking into why - not kicking it out the door anyway.
On top of this, there was a vertical slice/gameplay demo shown during a gaming convention that was nothing like what the final product was. Way overpromised.