Somebody: Of course, none of these countries had a robust bourgeoisie in command. It's not a new observation, but socialist revolutions only seem to happen in countries in transition to capitalism. To some extent, I feel like neo-liberalization and democratization in the former Third World has been about the installation of fully bourgeois regimes. It's difficult to see how any of them will be dislodged. Notice how the abortive Maoist revolution in Nepal was combating an anachronistic monarchy. There are very few of those left.
I agree with Wojtek that "Only under capitalism -which revolutionized the old economic order - the idea of radical systemic change "trickled down" to the masses." But, if that's the case, and if capitalism is now so venerable a system to no longer appear revolutionary, then what is there left to trickle down except the lack of faith in any alternatives? If most past class struggles consisted of peasant rebellions that led at best led to new dynastic regimes, then we can say that the revolutionary moment from roughly 1790-1990 was an exceptional period, and that we've returned to the historical norm.