Historically...

The Trans-Mexican Volcanic Belt, also known as the Sierra Nevada to locals around the area (translated to Snowy Mountain Range), was not something that sprouted out of the ground in one day. It may be a little obvious as to how the Belt was formed, but i will explain the historical process of how it did anyways.

First, like most mountains/volcanoes around the world, the Trans-Mexican Belt is a product of tectonic plate movement and volcanic activity. With most tectonic activity there are some results form that movement, such as, earthquakes, reefs, islands, and in this case volcanoes that run across ntire countries.  During the late Miocene period, there was a great amount of 'volcanism' and tectonic plate movement that ran across Central Mexico, from Mexico City to the eastern coast of Mexico; more specifically the period from 11-7 ma. The activity that occurred during the Miocene period resulted in a numbers of mountain ranges and volcanoes that inhabit the skyline of Central Mexico.

The Belt extends across Mexico nine hundred kilometers, running through the Mexican States of Jalisco, Michoacan, and Guanajuato. The Belt also has the two greatest mountain ranges in Mexico, which are the  Seirra Madre Occidental on the west side, and to the east the Sierra Madre Oriental. Even more so, these mountain ranges meet with the series of volcanoes that make up the Trans-Mexican Volcanic Belt. The volcanoes include the Pico de Orizba (which is the highest peak in Mexico), Nevado de Colima, Nevado de Toluca, Paricutin, Popocatepetl, Iztaccihuatl, Matlalcueitl, Cofra de Perote, and Sierra Negra. Within the nine different volcanoes that make up the Belt, some are active and some are dormant, which make the lands around the volcanoes very vulnerable.

More importantly, though, it is because the number of volcanoes that create such different terrain across the Belt , that result in different climate and biodiversity; such as forests and species.

Reference Link:
 http://www.sciencedirect.com/science/article/pii/S0040195199003108
http://en.wikipedia.org/wiki/Trans-Mexican_Volcanic_Belt