The reason has to do with the mindset of the city these teams have played in. When a Minor League team leaves town, the local government will now have to consider... what do you do with an unused ballpark. The team is gone, might as well put the stadium back on the market and get the real estate back into circulation. When the ballpark never sees professional play then the ballpark is just a ballpark, meant to serve the community. It could live there indefinately, flying under the radar.