As previously said vague terms need defining, so I just going to presume that "bad" means REALLY bad, like I really regret buying this sort of bad.
The thing about video games is whenever we play them we're looking to, well play something. If we were looking for a good story we look for a book, or a movie, etc. But for video games we want to play for fun, and if the gameplay is bad that means we're obviously not having fun and will ditch it for something else. Not that story isn't important for video games since a good story can enhance the experience by 100, it's just if the gameplay sucks bad enough for people to not want to play it then the story really can't do anything to help. It would be a case of 100 multiplying a 0, you still end up with a zero.
To put it in another way I'm guessing we all had those few games where we weren't having fun, said to ourselves "I don't want to play this anymore" and never touch them again. Those games could've had great stories attached to them, but we weren't willing to continue doing something we don't want to just to find out.
I'm sure a a good story can save a mediocre game, like something tolerable but not great. But if the gameplay is bad enough to, well be called bad then chances are the story can't do much of anything for it.