LogFAQs > #962101181

Grass types, like bug, tend to be early-game types. They follow a similar format to gane levels, where the earlier levels are in fields or grass themes, while later levels take place in ice lands, fire lands, or places of darkness.

It's for the same reason that you tend to find Ice, Dark, Dragon, and to a lesser extent fire types later in the game, as they are considered more late-game types.

So early game types typically have more common weaknesses.

Recent games like Sword-Shield have moved away from this somewhat, but the earlier games followed this formula.