People choose to be NPs for different reasons and have lived much different lives than MDs before they complete thier education. For many MDs they enter thier career at a different point in life most in their early 20-30s with the expectation that moving goes with their career path. For many of my classmates who are well established in thier lives moving would mean moving thier teenagers to a new school, expecting a spouse to get a new job and buying a new home at the ripe age of 40-50 y/o. Lets get real NPs don't go to school to move around the country and chase a dream job most of us have a life already established and we are enhancing our personal career aspirations. Moving does not fit into the picture. For some people who are unattached to spouse and or kids relocation is a reasonable consideration.
Not to mention if we were going to be making 100-300k i would have a different perspective on moving cross country but for the money we make its not worth the trouble to relocate.