Because it doesn’t work properly.
Because Microsoft has no excuse for such shockingly poor software.
Because when I bought a game, e.g. Bioshock, I was not allowed to play the game I had payed for because GFWL didn’t work properly.
Because it wasn’t even able to update itself, it would not allow me to play until it updated, ok so update, update would proceed maybe to about 70%, then stall and never complete despite countless retries. I tried to download the patch manually that also failed. And I was not allowed to play until it updated but it was unable to update. Unforgiveable.
Because it kicks it’s paying customers in the teeth, I heard that pirates have none of these problems.
Because once, twice, thrice, ok many many many times bitten so badly I will never willingly use such a broken, shameful piece of time-wasting rubbish ever again. Game over.