The United States currently has Social Security agreements with Canada, Chile, South Korea, Australia and most of Western Europe."
The differences between this deal with Europe and with Mexico are that
1. The numbers of employees covered by European agreements are much smaller relative to the number of Mexicans involved. I don't think there are 15+ million Europeans potentially covered.
2. The Europeans' wages are much closer to US wages. There isn't a great windfall to be covered by the US plan instead of the European plans.
3. The number of Europeans covered by the US plans is probably much closer to the number of Americans covered by European plans. If we have 100,000 Brits and the Brits have 80,000 Americans, then numbers just about balance out and neither country is carrying much of a load for the other one. The Mexican government will not have to deal with millions of Americans on their retirement system.