I've been to Zambia and the short answer is basically they have 1) government corruption 2) and don't have anything else besides copper. So even though you have a participatory political system the copper companies get insanely good deals and nothing is spent on education for example.

Besides Botswana they are actually doing really well for a sub-saharan African country. The politics have been a mess for a long time, basically two parties have dominated politics for the past forty years(one after the other) so you had a quasi single party state but a reasonable rule of law to keep things from getting out of hand and a fair electoral system for the past twenty years or so.

I'm not sure I buy the claim that the government doesn't do anything. Basically, it was explained to me that no one wants to upset the factions with political influence. Paradoxically, Botswana has a much more closed political system yet doesn't seem to have the problems with corruption Zambia has.