The technology failed in the face of stupid behavior.
This is a general principle that is often discussed among people who deal with managing risk professionally. It is often referred to as "risk compensation". The primary thrust is that people seem to be hard wired to accept a fixed amount of perceived risk (very different real risk) so when a risk mitigation technology is deployed, the common response is riskier behavior. One of the more commonly discussed examples is a study from Europe that studied the effects of adding anti-lock brakes to taxis. Cabbies drove faster and braked harder in corners. If anybody watch Baltimore put a beating on New England last Sunday, you can compare the number of head-leading hits to those in the Rugby 7s tournaments. Helmets lead to riskier hits.
Locking folders may have the same exact problem. Given the lock, people may fall into the trap of using them in ways that are riskier than how they would use a slip joint.
This said, I do think locking folders are a place where conscious, deliberate care can lead to safe usage - safer even than a comparable slip joint. IMO, there are two ways to get a folder to fail. Plenty of discussion so far on the dangers of pressing too hard on a locking folder. Fair enough. But on the other hand, I've been in situations where I was working fast and using a slip joint and had the slip joint buckle when I've knocked the blade on something. Yes, I should move more slowly but its the nature of working sometimes. Think clearing out old plants from a garden.... In these situations, the blade isn't getting a lot of pressure, but the lock definitely makes the knife safer than a slip joint.
IMO, slip joints (and friction folders) shine when you want to open and close a knife quickly repeatedly.
I more or less think of locking folders and fixed blades are near peers. The difference is that carrying a fixed blade is socially forbidden in nearly all situations. But... you have to choose to not be stupid. There's just no cure for stupid.