Earlier quoted context omitted.
Western toilets seem to work reasonably well with septic tanks in rural areas. Those don't involve sewer lines or treatment plants.
The issue comes down to cost. As he mentions in the article, the goal is to get to 5 cents a day. Just a basic calculation, a “standard” septic system costs between $1500-4000 to install, and lasts roughly 25 years. Just doing the math on the low end costs brings you to about $0.16 cents a day, 3X the goal Bill Gates is advocating. Note: this isn’t even taking into account the cost of the toilet itself or any plumbin…
Basically you've got to learn to crawl before you can run a 10k.
edit: You can get someone who is completely illiterate and cut off from civilization to understand the concept of digging a hole for the shit to go in. This has the added benefit of not requiring advanced manufacturing technology or materials. they just have to dig a hole and move the outhouse top over to the new hole when the old one gets full. This makes them self reliant.