Many altcoins fork from Bitcoin. So their codebase is often not much different from Bitcoin. That's why its relativ 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 protokoll 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 invenst as an exchange.