Were robes a factor in the spread of Buddhism? 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?
In Theravada, robes may be made of six types of material, including linen, cotton, silk, wool, jute, or hemp (Mv.VIII.3.1). I've got a wool robe myself and it works fine in Canadian winters, so that shouldn't be a factor.

There may have been other geographic-based reasons, though; other aspects of monastic discipline might have made it difficult to practice the strict vinaya of the Theravada in colder climates, e.g. alms round, not-storing or cooking food for oneself, etc. Or, it may have been just by chance - kind of a "you go left, I'll go right" kind of thing.

One reason for Mahayana Buddhism spreading quicker might be the fact that it is generally-speaking easier to adapt to, since the focus is less on monastic discipline and more on engaged practice; in Zen Buddhism, for example, monks can drink alcohol and have wives, so much less adaptation is required for new practitioners. Tibetan Buddhism has spread mostly due to the humanitarian issues surrounding Tibet and the charisma of the Dalai Lama.

Another reason would probably be the Theravadin focus on attaining liberation in this life, a goal not easily accessible to the masses; the Bodhisatta path, while orders of magnitude more difficult in theory, can be relatively easy in practice.