The vast majority of cryptocurrencies fork the bitcoin codebase. Therefore they all have the same api. So all these exchanges have to do is plug the new copycoin into existing inferastructure. The few that have been developed from new codebase require the site operators to learn a new api. Which may or may not be a difficult task but either way its tempting to apply your time to just adding new copycoins that take no new learning when those are available instead.