Because you get what you pay for. Getting fitted is pretty serious stuff. If they do it wrong, you'll be in a world of hurt. Ive heard complaints about Supergo's fitting.

If you dont mind the drive, try www.velo-fit.com. He's in Brentwood. The guy's name is Eddie M. He's a coach and does fittings so its not just some bike salesman who read a book on fitting. He's also a writer for cyclingnews.com. He knows what he's doing. The price is steep ($200ish) but its worth it. He'll also install LeWedge on your shoes.