A lot of visits from international people are.... MPs are busy people. If you go to Burma, you don't have weeks and weeks to spend looking at the whole country. As well, journalists from major newspapers usually go to a couple of major cities, and Naypyidaw, and then go out. They haven't visited ethnic areas.
Probably also the Burmese would discourage that. I don't know how hard people have tried, but it would be interesting to see, if someone were requesting a visit to Kachin state or to Karen state, what that response would be.