Because it looks like slow and highly decentralized chain is a horrible solution for building an exchanges on it. Its like building a huge sea ferry on a small mountain stream and be surprised that its stuck. Fees will go only higher (with market pump and volatility increase) unless a scaling solution will be implemented to ETH. Scaling solution will lower ETH decentralization and change ETH into BSC that is scalable but not as decentralized ... which is pointless because we already have BSC.