Comment by SaxonRobber

Because it lowers the housing stock in urban areas. Wealthy travellers will pay more for accommodations than the local market, so apartments become airbnbs and investors gobble up condos and housing which they turn into short term rentals. The scarcity pushes up the housing prices, but it never balances out because the locals can never make enough to compete with wealthy travellers and are forced farther from the city. The end result is a hollowed out city core.