World Bicycle Relief explains that its Buffalo bicycles are designed to be “extremely durable to serve the needs of people who travel long distances over rugged terrain with heavy cargo in some of the world’s harshest environments.” With that in mind, simplicity and ruggedness are absolutely critical, and the redundant chain system is designed to provide a high/low gear solution that involves as few breakable, externally mounted parts as possible. These bikes are being delivered to places that don’t have access to bicycle shops or spare part overnighting, so making something that’s as tough and easy to repair as possible is an essential part of WBR’s job.
You have to put it into context: these bikes are going to some of the most remote, poor parts of the world. How many belt drive dealers would someone run across in that situation?
And even if they last 30,000 km, what happens when they finally do need to be replaced? You need a split frame, which this particular bike can’t do without madding massive costs and introducing more things to break.
A single speed bike chain is very easy to find, dirty cheap, and last a very respectable amount of time.
The people using these bikes are in a “if it isn’t broke, don’t fix it” situation, and the previous model of this bike (true single speed) was working great for them. This very simple upgrade gives them a considerable amount of versatility.
Adding one more gear to a single speed belt drive bike would just make these unaffordable for the charity giving them to these people. And it would likely leave many of them stuck once maintenance finally has to be done.