It isn't. The economy over relied on oil revenues, like so many countries socialist or not. Add to that the Chavez-Maduro government's taste for authoritarianism, which, c'mon lads it's South America it happens everywhere. See also: corruption.

But the key to it all going to shit is the outside world's reaction to Venezuela. Because Chavez got in with the Russians and Cuba, Venezuela got isolated from much of the world. Those aren't stable friends. Doing this also makes you enemies, like the Americans. So the Americans sanction your economy to death, making the economic crisis even worse. Then they also fund opposition groups, largely spearheaded by affluent white people who were always opposed to Venezuela's government even when it hadn't gone to shit.

When the Maduro government falls, the economic crisis will get mirculously better because suddenly Venezuela won't be sanctioned to death by the West. They'll also strip all the good things Chavez did, like the expansive wellfare state, and privatise Venezuela's key industries like oil.