Long story short. As a country with a strong tourist vocation, Mexico would be naturally inclined to offer visa free travel to most nationalities.

However, due to our proximity to the US, our government is pressured by unnamed parties -not- to offer easy entry for nationals of many countries as they could in theory use us as a transit point to illegally enter the US.

Hence, now things are simpler, if you have a US Visa you naturally do not fit into that risk group and are free to enter Mexico without a Visa. Sounds logical to me.