Ok, NASCAR guy here,
Many tracks across the country have racing experiences and driving schools where fans can either drive a race car or ride along in one while someone drives. The most famous of these schools is the Richard Petty Driving Experience, which is the owner of the school at WDW. They are the ones who run it.
Now when you get to drive, it's no passing, no going full throttle, its just going a decent but controllable amount of speed. They have this at Daytona and no one is going more than 140 mph. That might seem fast to you but that is a wide open track where during a real race, cars are going over 200 mph.
A ride along is just buckling into the passenger seat and going close to full speed since the driver is usually very experienced, sometimes even a former semi pro driver.
Now for WDW, their race track has been around for sometime but hasn't had any real racing series come to it in a long time. For that reason, many of it's safety features are extremely outdated, that includes barriers. Every major racetrack now has what we call SAFER barriers. These are energy absorbing walls that help cushion the blow when a car hits a wall. All WDW has is a concrete wall. So hitting it can be pretty rough.
Now to the last part. For this ride along experience, they were using sports cars. These models tend to not be as safe as say a stock car. Sports cars also don't normally race on oval tracks so they aren't as forgiving against concrete walls.
Depending on what happened in this accident. Any of these factors above could have come into play. But racing is a dangerous sport and anyone who gets in these cars needs to realize this isn't something not to take serious. I've been racing for 12 years and working in the sport for 3.
BTW this attraction won't return. WDW was already making plans to demolish the track so I'm guessing after this it will be closed for good.