Due to limitations with aircraft on certain routes over mountain ranges. For example Delta has a dedicated fleet of 757 that are used for South America. It has to do with supplemental oxygen that not all aircraft are equipped with. When flying over the the Andes and an issue occurs with pressurization there may not be a safe altitude to drop down to and land without supplemental oxygen. When I say supplemental Oxygen it is when an aircraft carries more oxygen then the stardard amount. For example if you are flyinf from NY to FL and you run into a problem you can easily descend to 10000 ft and land. When flying over the mountains you may not be able to descend to that altitude right away due to the height of the mountains. I hope this makes some sense :-)