If you look at the countries where Theravada Buddhism has historically taken hold (Sri Lanka, Thailand, Cambodia, Laos, Myanmar/Burma) they are all in fairly close proximity to each other in the southern part of Asia. Countries where Mahayana Buddhism has historically taken hold tend to be considerably more widespread geographically in Asia and include the northern regions of Asia.
I remember reading on the internet (so it must be true! ;-)) that the reason for this was that in the Theravada tradition it was required that robes be made in very specific ways and made of cotton. Cotton not being suited to colder climates impeded monks from traveling north. The article continued that Mahayana had more flexible interpretations of robes so robes were simply made of wool and other heavier materials as monks moved north to spread the Dharma.
Is this explanation considered true and accurate or is there another explanation for the historical movement of Theravada Buddhism to areas south of India and Mahayana Buddhism to areas north of India?