Perhaps because the past doesn't equal the future and the market has changed/adopted the tablet concept more widely?
Plus the Surface Pro is both thinner, lighter, much less expensive, and will probably be much more widely available than the old PC tablets of the past. If you compare a machine that weighs around 2 pounds, and isn't even a 3rd of an inch thick for $999 and compare it to an inch and a half 5 pound machine starting out at $1699, you'll see the Pro is a considerably more attractive buy.
The biggest problem with the PC tablets back then were that they weren't a bad idea, but the technology wasn't quite there to provide the best experience. We're closer to that now than we've ever been. Ultrabook hardware is thin and light, multitouch and digitizer technology is cheaper and more mature, and battery life is much, much improved in comparison. We've still got a little ways to go before I'd consider them all around perfect, but we're a helluva lot closer now than we were in 2000.