Ironwing summed it up...
Water treatment plants need: trained people, BEAUCOUP capital costs, BEAUCOUP operating costs.
To get the above, you need: schools/institutions to train the people, reliable taxation system to spread the cost out, a stable government to run all of the previous, a government that gives a shit and isn't corrupt as hell to make sure the previous get provided.
Also, you need to socialize people so that they know to avoid dirty water, and to have basic sanitation skills. The WHO has a blurb on this stuff, it's not just water treatment, it's also sanitation because people eat/shit/drink out of the same place, they don't have basic skills like "don't lick your hands after you wipe your ass and don't wash cause there are germs", they don't/can't shower regularly, have limited food safety training/enforcement, etc., etc.
And we haven't even talked about the infrastructure to get the water to the people...
If you want to help, see the links above. It's about ~$100 a pop to get household, point-of-use water filters into homes. They work a hell of a lot better than nothing, but I'd never drink from one - they do nothing for dissolved things (pesticides, chemicals) and little for heavy metals.