Strong US ties, high interest rates, currency of choice until recently for the carry trade, and a large oil exporter. Large moves in the peso are largely tied to the price of oil.

The past year it's gone from almost 21 to the dollar to nearly 18. Certainly sucks for me losing 15% on the conversation rate along with the general inflation that has happened. My food bill in Mexico is up like 40% over the past six months in dollar terms.