When an exchange wants to add Monero to their list of coins, it has a brand new code to work with considering most coins are forks from Bitcoin. Is integration of Monero on exchanges or with things like merchant services more complicated than Bitcoin because of a more complicated code or command structure? Could this be why adoption has been slower than we would have liked?
As I understand it, right now it's slightly harder due to the interface that Monero uses, but this is in the process of being standardised so it can be seamlessly integrated with exchanges just like Bitcoin.
This may be the reason that Monero hasn't been integrated with other exchanges, but in the near future this should be one less hurdle.
It's harder, for two main reasons.
First, Monero doesn't have multi-sig yet, which means wallet security is a tougher challenge.
Second, it's different, which makes it harder for any exchange to integrate. They already have infrastructure in place to integrate Bitcoin and Bitcoin-forked coins, but have to build new infrastructure for Monero. It's not as plug-and-play.