>Isn't the western region more mountainous and the eastern region predominantly flat lowlands?

What you're thinking of is the Yucatan, which is, indeed, a giant flat slab of limestone. But much of the eastern region is also, for example, the Guatemalan highlands, or the Peten jungle, and the terrain even in the northern Yucatan is still pretty rough to actually travel across.

This all being said, as I said in my comment here, it's mostly a myth anyway.