No compelling reason, in theory, but the reality is that forex "brokers" who accept Bitcoin are doing so because they're actually counterparty market-makers who trade against their own customers, and long and successful experience has correctly taught them that customers who want to deposit by Bitcoin (just like those who want "competitions" and "bonuses") are the ones whose funds they will gradually but consistently and reliably win from them.
They do their marketing accordingly, to attract these very gullible "traders" who typically don't even understand the difference between a genuine broker and a counterparty market-maker.
Genuine brokers don't accept Bitcoin account funding.
Like so many of the other things which people try unceasingly to explain to you in this forum, it's simply an indication of what kind of "broker" it is, and whether we should be using their "services".