Because evidently if any competition comes to the domestic aviation industry, US airlines are doomed.
Allowing international carriers transiting the US to do intra-city flights soaks up demand and lowers route profitability for busy routes.
Past practices were to balance demand across city pairs so as to avoid local monopolies (ie. Minneapolis and Delta/Northwest) and ensure universal service. Deregulation broke all of that, so we have price competitions at the cost of a race to the bottom in service and industry cycles of bankruptcy and consolidation. Long term, that means monopoly levels of service and pricing.