Many altcoins fork from Bitcoin so their codebase is often not much different from Bitcoin. That's why itsit's relatively easy to make all existing tools work with a coin forked from Bitcoin. Therefore exchanges don't need to spend much development time to implement themost altcoins.
Monero's baseMonero is thebased on CryptoNote. A, which is a new protocol and very different from Bitcoin, so in. In order to make Monero work youthey need to change most of yourtheir tools. That's a lot of development time youthe exchanges need to invest as an exchange.