By far: The leverage. No need to go to a prop firm. 24 hours six days a week. No SEC mandating silly rules. Being able to trade very small accounts. Can be long and short in "same account" same symbol. Useful for multiple systems, multiple time frames. Fundamentally driven. Highly amenable to systematic trading. Very difficult to manipulate if not impossible. Embeds "Pairs Trading" at its core. The minus? Wide spreads on many pairs Very difficult to get direct access because the need for a Prime Broker and the requirements to do so. This reason alone a FOREX prop firm would be welcome.
FIFO is not applicable to all countries so traders from many countries can make this idea into a good one if they use it effectively.