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.
of course because this is a matter of fees, and also bsc has made their market decentralized and many new projects have started to adopt bsc.
and previously the old project on the erc20 network had opened a new network for bep20.
for example 1inch dex who have created a network for their own bsc.