Close proximity, easily driven to by car, abundance of plane flights and minimal time zone changes.
Reasonable cost of living: e.g. gasoline is $2.70/gallon, housing prices lag behind comparable US/Canadian properties and staple costs (food and clothing) are comparable to US/Canada.
Mexican Peso historically tracks closely with US dollar (10 - 11 MP/US Dollar).
Social Security, VA, IRS and AARP report over 1,000,000 US citizens living in Mexico and 1,500,000 US citizen property owners. Over last 6 years 400,000 US Citizens have moved permanently to Mexico and another 300,000 have made an investment in real estate.