Many altcoins fork from Bitcoin so their codebase is often not much different from Bitcoin. That's why its 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 the altcoins.
Monero's base is the CryptoNote. A new protocol and very different from Bitcoin, so in order to make Monero work you need to change most of your tools. That's a lot of development time you need to invest as an exchange.
