[TimovieMan]

I agree with those stating it's a stylistic choice. It definitely changes the *feel* of the dialogue in an adventure game, and I reckon that's primarily the reason to use it.
Having an adventure game in the past tense implies that everything you do has already happened (like in a flashback or a story being told).

For Discworld Noir this was very fitting because that started with the death of the main character and then told the events of how he ended up dead.
I don't remember if the final chapters were past tense as well, though. Would seem fitting to have those in the present tense instead. But then again, that game was noir, and noir usually has a past-tense narrator...