The bottom line here is that this is what happens when you turn industry over to government. Pemex is owned by the Mexican government. They have zero incentive to maintain environmental and safety checks. Pollution is an externality. It profits them nothing, yet also costs them nothing. So they don't care.
It is no different than a government-run nuclear reactor such as Chernobyl or Savannah River. Government has no incentive to build a containment dome over its own reactor, so they don't.