It couldn't be the Wright Amendments doing, as it was repealed. Maybe something else?
United flew into both as of a couple of years ago when SWA made them an offer they couldn't refuse...$$$. United still has the DAL gates but leases them to SWA.
American had gates at DAL and during the Wright Amendment but was forced to turn them over as a condition of the USair merger, which is where VA and Alaska got the gate space.
Delta has gate space at both too.