The pretty obvious reason in my opinion: Ethereum's fees have been quite high for a while, and Binance Smart Chain(BSC)'s fees are drastically lower. And yes, BSC is like magnitudes more centralized than Ethereum, but it comes to show how a lot of people don't care that much if a certain project is decentralization or not as long as they can make their bets on certain tokens.
I really don't think centralization is an issue with BSC compared to Ethereum. Ethereum literally rolled back hacks and gave people their money back. There is literally nothing worse you could do from that standpoint, so no matter what the powers that be at BSC do they still won't be as bad as the Ethereum leadership.