We enjoyed spending the winter on the Pacific coast south of Mazatlan for 4 winters ending in 2010. The increasing violence around Mazatlan was one of the factors that ended our Mexican snowbirding.
If it's a fact that the RV crowd is getting older the other possible factors that make Mexico undersirable are in no specific order:
Unavailability of ULS diesel for newer diesel equipment. A friend who still goes down is limited to going as far as San Carlos as that's how far a tankful will take him into Mexico and back. Not worth the risk and hassle of warranty/mechanical issues for lots of folks.
Border hassles/delays are another potential road block. Why put up with long delays, communication problems, hassles bringing in your play stuff and the requirements for holograms and bonds on your CC when you can move to and from Canada and the US much easier?
Perhaps the biggest negative is the unavailability of first world RV parks. Having to worry about quality (and quantity) of reliable electricity, potable water and a reasonable RV environment is definitely a negative for the older folks. El Mirador in San Carlos in it's heyday came closest to a first world RV park that attracted the RV crowd. I guess younger folks from Europe in Expedition RVs like the challenges of Mexico but I think a lot of Rvers north of the border have forsaken Mexico for the reasons above.