I think the biggest issue is that under Part 91, pilots are free to take a look/see on an instrument approach even if the reported weather is well below minimums. When you combine that with good, old "get-there-itis", you have a lethal combination. The pilot makes one pass, doesn't make it, but sees enough to think that another try may be more successful. Then, on the second pass, s/he presses a little lower than allowed because s/he sees a little ground below and thinks that another 50 feet will be the difference, and instead it's fatal. Pressing on with low fuel so you arrive without enough to divert somewhere is another potentially fatal mistake -- lacking any other option, the pilot presses the approach below published mins because s/he has no other choice but to run out of fuel and crash, or does in fact run out while trying again and again.
A classic example of the first type is the case of the President of the Flying Physicians Association a long time back when they were having their annual convention in Montreal. As a FPA officer, he planned to arrive a day early to help set up, but the weather was horrible -- ceilings/visibility below even ILS mins. He missed the first, but feeling the need to get there, tried again, went below DH, and crashed into the approach lights. BTW, the theme of that convention was "Why do so many doctors have so many accidents?", and yes, he was flying a Bonanza.
The classic example of the second is the
Avianca 052 accident, in which the crew flying a Boeing 707 pressed on to JFK despite weather at/below mins and using up all their divert fuel in a series of ATC holds, and chose not to land short to refuel at Baltimore or Philly where the weather was much better. They ran out of gas while coming back for a second approach after missing the first.