Presumably you want the US as happened OTL. If not, the answer is easy: have the US fall apart upon independence. OTL Mexico is automatically a better option, or at worst, no worse than the various failed or bickering republics of the 13 republics.
Right around mid 1700's have a proactive Spanish King who decides a prince should become master of the land. A (decent) Prince turned King will highly likely create an atmosphere which will foster cohesive nationality. the move needs to overcome the existing attitude of Penninsulars are the only ones worthy of running the territory. But if the Prince does that, and manages to create a decent governing structure, North America is the Mexican Oyster, regardless of what the US does.