Probably because there are no old Pokémon in the main game, so type distribution is very low. There are very few Water or Flying Pokémon throughout the whole game.
But to be fair, most of them aren't awful. They're pretty useful. They're just not very varied.
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Okay, I rolled a 14. What's that mean? Hsu
That you're a cheater. This is a 12-sided die. Chan