Because when it comes down to it we have three components, only two of which can be grasped by mere mortals.
Price, Time, Quality. You can have a a great game, in short time too, but the budget will be astronomical.
You can have a great game on a reasonable budget, but the time of release might be delayed several times.
Those are really the only two scenarios that keep a good game from being a great game. Either the money runs out, or the time does.... Then you have those games whose ideas are worthless to begin with.... Nothing can help those games. Me and My Pony? Seriously...