Oil prices dropped from $100 a barrel to $40, their economy relies almost solely on oil exports. There's some amount that is explained by corruption and poor investments over the past decade, but primarily it's the oil price.
Beyond that, Saudi Arabia has substantial infrastructure development to pull oil out of low-value land. Venezuela has ongoing fights over which reserves should be recovered (it's under valuable rainforest and watersheds) and excess spending has steadily undermined infrastructure - their break-even price is ~15% higher than Saudi Arabia's.
So: Venezuela sank their wealth into some mix of corruption and admirable initiatives that nevertheless haven't paid themselves back. Saudi Arabia saved the money and drove down production costs.
https://www.theguardian.com/world/2013/jan/01/saudi-arabia-r...