According to a source close to the band:

Passport issues: Ukrainians are high on the watch list apparently. Makes it difficult for them to get into the USA. This also may explain why he "can't be in a touring band."

No Money: Denis is unable to come to the USA cause he's broke which surprising considering how many copies The Black sold.

The Document Speaks For Itself: Denis is under contract to Sumerian Records for two more albums. Who knows how these will be fulfilled.