Whatever you and I say about this are educated guesses.
He wasn't the president. My educated guess is that with Cuba, Nicaragua and Venezuela, together with China support cartel become too powerful. They systematicallly buy politicians in México, Spain, Colombia, probably Brazil and Argentina. They expanded too much. But again, it is just speculation.
I'm not Venezuelan, but I am Brazilian, so I consider myself closer to the problem than people outside of South America. If the cartels were the real problem, the US would have invaded Mexico or Brazil a long time ago.
Maduro was not fairly elected, it was a fraud, but he was the de facto head of state of Venezuela.
The whole cartel excuse is just a sham in my opinion, it is all about power, sending the example and getting the oil. Maduro, and Chavez before him, challenged the US grip on SA, and actively fought american interest in the region.
He wasn't the president. My educated guess is that with Cuba, Nicaragua and Venezuela, together with China support cartel become too powerful. They systematicallly buy politicians in México, Spain, Colombia, probably Brazil and Argentina. They expanded too much. But again, it is just speculation.