Quite simply bitfinex has been around a long while longer, that's the main thing, second to that I think the actual bitcoin trade will always be bigger than the futures trade, or maybe not but at least for the foreseeable future. Most people have no need for futures as they are speculating that the bitcoin value will increase so they can simply buy bitcoin now and hold it instead of using a futures contract. It's also a new industry and in time it will grow, especially as it begins to be offered in more places and as more people begin to understand the use cases.