Because decentralised exchanges are sadly not ready for large amounts of money. I'm not defending centralisation, I hope we move towards that way. My point here is that bitcoin can also be used in decentralised exchanges, you don't need monero.
Of course, Bitcoin can be used on a decentralized exchange. However, most Bitcoin is obtained through centralized exchanges with no-year customer, and therefore reduces the anonymity of people who use decentralized exchanges, making Monero more of a need.