Malnourishment is a big part according to the WHO [1]

I’m no expert but I also suspect access to medical care is also a big part, as typically only active cases spread the disease. If you are symptomatic and get some antibiotics, you might not spread it to very many people. If you’re symptomatic and go without treatment for ages, then you might spread it to loads of other people.

As we learned during covid, small differences in R value massively change the prevalence over time.