I suspect there are a lot of factors at play.
In developed countries, there is generally a good postal delivery network to all places, widespread internet access and the majority of people have credit/debit cards that make online payments easy. I don't know to what extent those things are true of developing countries. I am sure that in bigger towns and cities, there are substantial numbers of people for whom those things are true, but equally I would expect that in rural areas they would be less common, and in poor parts of bigger cities they would also be uncommon.
You've got to remember that internet usage in developed countries is a much more mature industry than in developing countries. Using figures from 2011, about 75% of people in the developed world were online, compared to just 25% in the developing world* – in the developed world, we were there 12 years ago. And for people to want to use internet shopping takes not only ability but also confidence – for a lot of people, particularly older people or those from lower socio-economic groups, it takes a few years of being online before they will really trust internet shopping, and internet usage in most developing countries has been slow to take off so there are fewer people able and ready to use it.
And then there's the question of what people are buying. The majority of internet purchases are not what you would call "essentials" – they are books, CDs, DVDs, holidays, plane and train tickets, large household items and appliances, gifts etc – in developing countries where a lot of people have barely enough money to keep a roof over their head and food on the table, spending on these kinds of items is going to be low anyway.
- And that 25% hides a huge variation: more than 20 countries in Africa plus a few others around the world saw fewer than 5% of people using the internet, and even with a burgeoning economy India had only 12%.