The interface is a bit clunky and not that intuitive. Also when all the volume is already on Polo, you don't have a lot of reasons to use other exchanges. For example I may only use Bittrex for trading coins that are not available on Polo.
Well why i wouldn't trade with Polo (although i admit that they have a very friendly User interface compared to btc-e for example) is because it may as well be the next Cryptsy ?