That may be true in some cases, but I suspect the real reasons have more to do with the level of fraud on both sides of the transaction(s) (i.e.: seller really did not deliver; or buyer claims the seller did not deliver even when they did), as well as liability in an uncertain regulatory environment. Some countries have accepted (some) crypto currencies as legitimate forms of payment, while others have taken a "wait and see" stance, and others have banned it outright.

From the perspective of the banks, there just is not enough reward to justify the risk at this point in time. That could change in the future, but for now, all we've got is a big can of nope.