because the liquidity is crap. Let's take the example of the British Pound. Trading a 10 lot in the 6B futures is $62.50 per tick often you cannot get that relatively small amount off at a single price level. That's the equivalent of 6 retail lots in spot FX. I believe you can clip off 1000 retail lots in spot FX with virtually no slippage, certainly a couple hundred no problem. Granted the euro is a little thicker in the futures and the $12.50 tick size helps but still anyone trading decent size will run into problems quickly with futures.
So if I want to trade FX, trade with IB FX is a pretty good choice and better than CBOT future?