Why haven’t developing nations developed yet, how long will it take before they do?

It appears some ”developing nations” have yet to finish developing, or developed to the point they can shed their developing nation moniker. Why have some nations made it to iPads, tablets, robots and satellites, etc. while other can hardly manage running water and functioning sewers? Is it education, even when they send many people to first world nations to get educated? Is it lack of money or mismanagement of the aid they get or the GNP of their nation? What is stopping developing nations from developing?