Maybe the reason bitfinex is so popular is because it allows users to 'borrow' bitcoins so that other traders can use these to trade on margin and make a profit from borrowing their bitcoin.
kraken do margin too. maybe they do less of it, and customers themselves can't lend on there as far as I know. I guess it's the liquidity people liked about bfx but the same can be replicated elsewhere if they had the gumption to go to a place that didn't screw them.