This is an easy question to answer. We like to winter in Mexico because we live in the S.F. Bay Area. Our weather is nearly perfect eight months out of the year. We do not have to deal with bugs and humidity plus summers at home are great for boating, fishing and gardening. So why not stay home when life is good and vacation in Mexico during the blah, rainy months of Jan - Mar.
Because we travel with a truck-camper, we do not have a "toad". Most Mexican campgrounds are within easy walking distance of bars, restaurants and shopping. So, if you get a little tipsy, you can get home by "shank's mare" or a short bus/inexpensive taxi ride. Whereas, most American and Canadian campgrounds are usually miles from amenities and entertainment.
(Recently, we attended the California State Fair in Sacramento. We were unable to get into the Cal Expo Park so had to settle for the next closest park, a KOA about ten miles - three bus changes - and a long walk each way. We will not do that again.)
We concur with the reasons others have listed: friendly people, culture, historic villages, interesting markets, great food, warm weather and beautiful sandy beaches. And safety - we wander around the barios of villages and cities alike. We have never felt unsafe in off-the-beaten-path areas of Oaxaca, Mazatlan or any other large city we've visited.
And, frankly, RVers in Mexico are a heckova lot friendlier than you'll find in a typical American RV park. Having friendly neighbors really adds to the enjoyment. Everybody is helpful and willing to share their experience and recommendations.