why don't exchanges trade xrp?
Primarily because XRP isn't a cryptocurrency forked from BTC or LTC (and thus as easy to integrate, many exchanges just clone the code and point it at a new deamon for X new coin). Ripple is an entirely new system which compliments all currencies, allowing them to be managed and sent within and outwith the ripple network, both directly from person to person, or via traditional orderbooks supported by various exchangers, and XRP is a utility of that network. Any exchange which supports ripple, by default supports every currency they choose to support. Further ripple allows currency backed by an issuer to be traded with the same currency backed by another issuer. e.g. it allows MtGox.USD to be traded for BTCe.USD or Bitstamp.USD - or literally any combinations of currency you can conceive.
XRP is supported by some exchanges already, and being added to more, but really ripple is a newer approach on which you could build entire exchanges, or indeed you could just create your own exchanges within ripple.
