why not possible to control vector transmitted disease in the undeveloped parts of the world?

Answer 1

Because it's really hard to control the vector. Vectors, often insects, can't be fully eradicated because if you take care and wipe clean of vectors one area, they just move in again from another area to replenish the freshly eradicated areas (think about DDT spraying in the 1960s worldwide, much good it did, now malaria is coming back 40 years later).

Imagine this situation in a country where 90% of the population lives in rural areas, often surrounded by woods, swamps or rainforest; how can you control the vector there?

The solution however is rather easy given 1 of 2 conditions.

If there's no intermediary host (in cases like dengue fever for example), all you need is to control the disease at the host level. If there's no infected host that the vector can pick up the disease from, then problem solved, let the vector run amock.

But if there's an intermediary host (in cases like yellow fever or malaria for example, which have several), it can get tricky, so then you must do everything possible to control the vector and the intermediary, but catching and inoculating the intermediary hosts can get quite tricky. Obviously, often this is not possible because of geography and social/economic conditions. And if the country is poor (like countries in central africa) then that's a battle that can only be won at the population level, by education and training, neither of which is being applied right now.

Answer 2

Drug availability. Too many vectors. Too many people in a small area.

Answer 3

poverty, lack of education, war, and availability of specific control methods