I think first and the most important factor not only for volleyball but for all sports and art is economic factor. If the economic state of people is not good, they won't think about volleyball or their NT Also we must take into consideration some local factors.
I am sorry, but I am not totally sure about that. Economy can play a huge role, but it does not determine the fate of a sport. Economic factors can speed up or delay the development of some sports, specially for those sports with highest technical requirements. For example, look at Dominican Rep. they have a good volleyball team and DR is one of the greaters "suppliers" of baseball players to the Baseball Leagues here in the USA, and the country is not even closer to be a rich country.
Or take for example the ex-former Soviet Republics, specially the ones in Central Asia, they are not among the wealthiest countries in the world, but they are developing good volleyball teams, boxers, and wrestlers.
Only agree with your last comment, there were not big differences among them, but the countries were economically collapsed.