One obvious answer: it's cheaper. While prices vary from one place to the next, it is still possible to live modestly on $700 a month in some areas without having to move into someone's garage. For those on a more extravagant budget, you might be able to buy your dream home on the beach for $900k instead of $2 million. Food, clothing, gas and electronics tend to cost the same or more as in the US, but the cost of housing and eating out is usually much cheaper. Utilities are cheaper and house taxes are very low.
Another advantage of retiring in Mexico is the low cost of labor. It is easy to find help, whether you are looking for a maid, a gardener, a caretaker or someone to help with your home repairs. I might add that like everywhere else, home repairs are not likely to be without some problems and cost overruns, but the handicrafts, tiles and artwork in Mexico are so attractive that remodeling and decorating a home is a real pleasure.