One reason I haven't seen mentioned yet is that countries like India can be overwhelmingly mobile-dominated (as in, few people own desktops).
I never had data on India but in places like the Philippines or Indonesia (with similar GDP/capita), your buyer's only internet access might come from his (very basic) smartphone and I sometimes saw > 90% mobile visits. At a previous company, we saw our Indonesian traffic soar after we fixed a bug that prevented the site from displaying on Blackberries... somehow an incredibly popular brand in the region.
If 90% of your buyers are on mobile, you might as well become a mobile company, if your IT resources are limited.
The gap between mobile and desktop is closing though, as companies like Xiaomi keep releasing ever cheaper, more powerful and larger devices in those markets. A Redmi (1, 1S, 2...) costs a tenth of an iPhone for 80% of the functionality.
The reach of mobile devices never fails to impress me. We went horseriding a few hours drive from Jiuzhaigou, and after a day of climbing the mountains on horseback, sleeping on the sloped floor of a tent (heated by a dried yak dung fire, of course), it was quite surreal to see all our guides, on horseback, playing mobile games and happily texting away on their 5" smartphones - even as they were herding yaks...