Pretty sure DL demanded it as part of the substantial investment they injected into the airline. Now why they didn't in turn join SkyTeam is another question. Now they are just kind of lingering out there, with reciprocal benefits for DL elites, and several remaining partnerships with former OW partners, but notably missing is AA, no surprise there as that directly competes with DL's interests where the other remaining OW partnerships do not, or at least do far less.