Partially it's population explosion, if you're already running near your max and somebody gives you a piece of tech that let's you squeeze out 20% more food from the same land during ten years of upgrades you basically achieve nothing in terms of standard of living, since the pop will swell up 20% during those ten years... though I feel obliged to point out that the standard of living in those places has almost universally risen a lot in the last century. If your culture trends to max out it's pop, because it's all about having lots and lots of kids, you can make a bigger pie only to find out you've got more mouths to feed. That's why a lot of the efforts in those places seem so futile, you upgrade some village and by the time you're done, you've got twice as many people there. Also the Peace Corps isn't exactly the model of effectiveness either.

But you really have to be to those places - and I'm sure they're all different - to appreciate the problems. When I was in Iraq most of the villagers still lived in mud huts... and that's not sneering metaphor... wasn't exactly what I'd call a strong work ethic, feeling of industriousness, or motivation to improve their situation either, but that's different in some areas and other countries are their own stories. Probably more than even the population expansion rates is the actually countries themselves. It's hard to build up your own little farm when the local tribal chieftain might just decide to come take your stuff just because you have it, or your village to work together to build up only to get smashed and looted by jealous, greedy or paranoid neighbors or rulers. It's hard to get a loan from the first world not because they don't think handing someone a few thousand bucks isn't a good investment, our bankers have brains and most actually have hearts too, but they can't cut loans to individuals for small change like that, besides the high risk of default by local pillaging there's no incentive to flying to the middle of nowhere and hiking out miles to loan a farmer $10,000 since you probably spent that just getting there and back, doing a whole village ain't much better, and it's hard to do banking in places where there aren't exactly photo ID's and such. We can loan to countries or big wigs, where it might trickle down, and sometimes genuinely does, assuming they don't get knocked over in the next coup or take the money and run. It's a stability issue, not a western greed thing. China's hardly the lion of economic success a lot of people talk it up as but they've made massive progress almost entirely by being stable. We don't really like doing business with them but we know if we put in an order for 10,000 widgets to a factory there they will probably deliver on time as opposed to having the factory get torched by the rebels and state thugs. It's not complex, and it's nothing to do with inability, just instability. Just talking about it or hearing about it doesn't really paint the picture in its fully depressing glory.