>>50312157So I don't know the history of competitive RBY / Gen I at all.
I have no clue how the earliest top threats were decided upon, how things have shifted over the years, or how, if you were to rebuild the contemporary meta from the ground up, what mons would be the most foundational around which the rest of OU would be decided. I have zero clue, though all of that is critical for understanding what makes it so good.
But what I can talk about is what it is right now, even though that doesn't really address your question.
Strong special attackers are very dominant in RBY. The majority of the ones in OU are part Psychic type, although not all of them are. Each one actually does a pretty good job of checking at least one other Psychic type, but there's none of them that can check all of them at once.
That's sort of where Chansey comes in. It does an okay job of checking all of them at once, depending on your set, your positioning in the game, and your luck. It's not a hard wall, but it's the most reliable option you'll have.
Even though it is possible for these special attackers to overwhelm Chansey, it's not an easy nor enjoyable task. That's where your physical attackers of the tier come in.
Something important to notice is that OU only has 3 physical attackers in the whole tier: Tauros, Snorlax, and Rhydon. Outside of that, people sometimes use Persian---but beside that, physical attackers are slim pickings and rare in competitive play. This can be summed up with their issue of having bad movepools, disadvantageous typings, and perhaps most importantly low SPC.
Rhydon has a place in the meta, but it is very fragile in this special attacking tier, and slow. Which leaves Snorlax and Tauros as the only consistently viable options. You need either of those two or Persian for a particularly viable team.
And that's how the meta ended up with The Big Three Normals: a lack of other options for the roles needed.
