LogFAQs > #982881228

I think there are a few factors to it. First, let's get the obvious one out of the way: Game Freak isn't exactly a particularly competent developer and more over they aren't exactly aiming for competitive balance in their monster catching game that needs to be beatable by barely-literate children. So from the jump this is an uphill battle.

It's made even more difficult by the fact that the type effectiveness and resistance has to at least sort of make sense, because again, this game needs to be understandable by children. Types are gonna have logical weaknesses. And unfortunately, plants have a lot of real life weaknesses. So it gets the short end of the stick.

Finally, I don't think Game Freak ever intended for Grass to be a strong offensive or defensive type. It was the utility type. It had status powders and leech seed and what not. The problem is that it's utility aspects diminished as other types got more and more non-attacking moves.