Interesting, I'm just guessing. Since Dwolla operates in the US, from the recent regulatory announcement targeted at bitcoin(or any online currency), I'm just thinking that Mtg.OX not registering itself in the US, it won't be able to do business directly with US entities. The Ars article http://arstechnica.com/tech-policy/2013/03/us-regulator-bitc... would lead me to believe the money laundering is the most likely aspect to run afoul of regulation at the moment.

However, I have little insight, it just seems the most likely reason to me.