Exchanges such as Poloniex.com that trade in Monero are limited as compared to other cryptocurrencies. Is Monero trading more difficult to implement than, say, a Bitcoin-based exchange? What are the unique challenges required and technical steps needed for an exchange to adopt Monero?
Many altcoins fork from Bitcoin so their codebase is often not much different from Bitcoin. That's why it'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 most altcoins.
Monero is based on CryptoNote, which is a new protocol and very different from Bitcoin. In order to make Monero work they need to change most of their tools. That's a lot of development time the exchanges need to invest.
Monero uses a completely different codebase than bitcoin, and the codebase is poorly documented (though its getting better) . For instance, Monero's software architecture is strikingly different - there is Monerod (aka bitmonerod), which is responsible for the p2p network functionality, and then there is monero-wallet-cli (aka simplewallet) which performs the wallet functions.
Add on top of that is the fact that to function as an exchange wallet, the exchange operators need to understand and integrate payment IDs, which can make things difficult.