Digging a hole is more challenging than you think.
It has to be deep enough, and far enough away from a home, road, water source, food supply, etc. Not all communities have a lot of extra land, so you're talking one or two communal toilets, which need to be large to support a large population, and someone has to maintain them.
If you have the land, you have to have money to construct them, as well as labor. Many households can't afford the $1 (that's one US dollar) construction cost, or the household simply can't perform the labor. In some cases, villages will come together to help fund and construct pit latrines, but these are the exception. Most just put up with the shame and disease of open defecation.
Once you do have them, you have cultural issues like India's manual scavenging problem (http://www.thealternative.in/society/manual-scavenging-thoug...) not to mention you literally have to go out and inform millions of tiny remote villages about why and how to change. It's very hard to get people to change.
Assuming outreach of one week per village, for the 600K censused villages in India, that's 11,506 years to just inform and convince them all to build pit latrines. Even 50 outreach teams would take 250 years. With 500 teams you can cut it down to 23 years.