Due to the Sierra Madre mountain ranges, which crisscross the vertical axis of the whole country. Also both join in central Mexico along the "trans-Mexican vocanic belt" which runs from east to west.
The three Sierra Madre (Occidental, Oriental and del Sur) mountain ranges difficult road movement in Mexico.
South east, past Central America.
http://www.seniorcitizenjournal.com/travel-articles/senior-travel-Mexico-central-America-and-bus-tours/. this link has information on central America senior trips.
because there was no place and Mexico is small
I do not know except is it cheap. But you can go farther south in central America and in the caribbean just as cheap. Mexico is filthy and unsafe. Travel elsewhere!
It depends on the route you take, but you need to travel across most of Central and South America.
mountains blocked travel between areas
mexico's central plateau makes up more than half of the country's area. most of mexico's people live here. however, the surrounding mountains make it hard for people to travel to and from the central plateau.
no unless you want to risk going back to prisin
From the US? Yes it is, if your destination is Mexico (capital: Mexico City), the country which lies south of the United States. If you mean New Mexico (capital: Santa Fe), that would be considered as domestic travel.
why was travel across the Sahara desert difficult
If you mean NEW Mexico, no; it would be a domestic travel. If you mean Mexico the country, yes, it would be international travel.