Did you see someone create a pool and stick 1 billion tADA in it. When testnet first came out. (Probably an exchange)
Saturation back then was next to nothing 30million maybe, or less. So that 'whale' destroyed himself, it had to move all that tADA as it barely recieved any reward.
People in pools like IOHK1, which is unbelievably over saturated. People are picking that pool because they see it's IOHK's pool, but they are just condemning themselves to no rewards.