As was mentioned, the track was a joint project between Disney and the parent company of the Indianapolis Motor Speedway. The primary intent was to use it as a venue for IndyCars as part of the IRL series. But the problem is that the track is a rather "short" tri-oval with three turns instead of the normal four (though technically a true oval only has one LONG turn on each end). This meant that the WDW Speedway had to squeeze 360 degrees of turning into only three turns. Tighter turns means a greater chance of a car impacting a wall in more of a "head on" orientation. There were two notable IndyCar driver injuries at WDW Speedway. The first was Davy Jones (not of "The Monkeys"), who suffered a serious neck/spine injury in 1997 that nearly decapitated him (by some accounts), but he survived and after a long period of time wearing a "halo" device resumed his racing career. The worst accident was Sam Schmidt, who also suffered a practice crash at WDW in 1999. He wasn't so lucky as Jones. Schmidt suffered a spinal fracture that left him a quadriplegic. However, Sam's done a lot since his injury. He's a race team owner now, and last year I watched him drive a biometrically controlled car around the Indianapolis Motor Speedway at over 100 MPH. Both of these crashes were before common use of the HANS device on helmets that limits the forward extension of a driver's head to the point where they risk neck injury or a basal skull fracture.
Watch this video and see how much time each lap an IndyCar driver had to turn the wheel during a lap a WDW:
The track also was never fitted with a "soft wall" SAFER barrier.
IMS and WDW entered into the project because WDW wanted a race there in late January as a means to increase crowds during a slow time of the year. But after a couple of years, IndyCar expressed a desire to move the race later into the calendar because a January race left an almost two month gap before the next race. WDW didn't want to move the race to a "busier" time of year for them, so the races were stopped.
Why is it closing now? My guess is that the track may be in need of a resurfacing, which if true would be quite expensive.