Largely due to the environment they are placed in. Some countries are corrupt beyond belief and have little to no assets. The combination of the two, plus poor education / insufficient nutrition for development creates a vicious and ongoing cycle that is nearly impossible to break.
this is not enough. You missed out the genetics of intelligence and if we are honest, we see the evidence of natural limitations in others and ourselves. Some people are less intelligent and therefore, even now, achieve less than more advanced cultures, in most modern countries, maybe all of them, no matter what we do.
It is wrong to ignore this reality. Some people are poor even if you feed them, give them healthcare, education, money etc. It does not matter.
It is much like seeing children in adult bodies.