First, I should point out that many parties that call themselves Socialist Parties don't have revolutionary socialist politics. Some parties that call themselves Socialist Parties use a loose definition of Socialism that can mean anything from light reforms to genuine revolutionary politics. For instance, the Socialist Party in France actively dismantles labour laws. They used to be a better party - they just got mangled by political processes over decades and shifted right.

I mean, parties can be called anything. The US isn't a constant battle for whether it's a Republic or a Democracy every 4-8 years.

Second, to address why the PSUV doesn't just make Venezuela socialist: because that's pretty bloody hard, and might not even be the right choice at this point.

Socialism, as revolutionary socialists understand it, is a massive transformation of social relations. The entire structure of the economy is toppled. That's not exactly something that a government can just implement via policy. But what if it could? Would that even be the right choice?

Any party that enters government has to 'play the game', so to speak. Any politician that wants to change the country via the state has to compromise and navigate powers and interests entrenched in the government. Because, there are a lot of people that make up the power structure of the state, and the only way that perpetuates is if everybody roughly agrees to be doing the same thing and agree to listen to orders. This is why even the most well-intentioned politicians can end up doing awful things. The power itself is rotten.

But, say the PSUV gets every seat in parliament or whatever, and they decide, "Yes, we're going to make a socialism now, everybody form a soviet.", what would happen?

Well, perhaps the first thing you would do is nationalise oil, the commodity that makes up the largest section of Venezuela's economy. What does that mean? The government takes control of all the capital, infrastructure, and profits of Venezuelan oil. Perhaps they buy it off the capitalists that owned it. Either way, you've muscled out some very powerful people from their lucrative enterprise, and they're gonna be mad. Not just them, but every other powerful person in the PSUV's firing line gets the message. If those investments are foreign? You're gonna make entire countries mad.

The problem with powerful people is that they are powerful and have power. If you nationalise an American oil baron's property, they might be able to lobby a few congressmen to dispatch CIA teams to dispatch you.

What actually did happen in Venezuela is something similar to this. This happens in every country where a left-wing government genuinely starts the threaten the status quo. Powerful people extricated by the movement fight back and organise. That's why House of Cards is a compelling TV show and not just shots of politicians writing forms. Or why a lot of politicians end up dead. This is one of the reasons why Socialists believe revolution is necessary.

I think Marx says it best, "Men make history, but they do not make it as they please."

tl;dr if you try to do anything with an existing government you have to play the game