It might seem like they should have more redundancy - one cable broke, then a second, and just a week later the whole thing falls apart - but for structures held in place by tension it doesn't work like that. A bicycle wheel has redundant spokes: you could design a wheel with 66% of the typical number of spokes and it would work OK. But if you do break a spoke, the wheel will start deforming straight away. If you can…
This is not a matter of incompetence. They even had auxiliary cables installed some time ago, one of these was the first to fail. All well-known best practices don't matter if structures cannot be maintained because the governing bodies deny the necessary funding for maintenance/repairs/operation. In a sense Arecibo was knowingly "given up".
But it was installed on the cheap: if they'd had more money they would likely have paid for more diligent engineers. After all, they built it pretty well the first time round. So at some level, yeah, the root cause is a lack of funds.