The short explanation is:
The band doesn’t have any control over which countries they play on tour. It all comes down to the promoter of the tour to decide which shows are played, and the decision is usually purely financial. I’m sure DT would love to play in every country on Earth, but sometimes it’s just not possible. It has to make sense logistically (they have to be able to get their gear there). It has to make sense economically (they have to be able to get there without paying to play, without financial risk).
So the promoters and the management has to put together a tour itinerary that makes sense both financially and logistically, while at the same time respecting the bands wishes for required down days, for required length (the band does prefer not to stay away from home for more than 5-6 weeks at a time) and it has to be secure (the band can’t travel to high risk countries).