At the time, the assumption was that OV-101 (Enterprise) would be refitted to fly in space. However, results of tests on the 'Structural Test Article' STA-99, which was only ever intended for ground-based stress testing, showed that the design of the Orbiter needed to be beefed up. OV-102 (Columbia) was still in construction and so could be modified, but it became clear that OV-101 would have to be dismantled and substantially rebuilt, and in fact it would be easier to rebuild STA-99 as a flight Orbiter. So STA-99 became OV-99 (Challenger) and OV-101 sat in a hanger for twenty years before becoming the star exhibit at the National Air and Space Museum's extension site at Dulles Airport.

(If you're interested in the history and development of the Shuttle, I cannot recommend this book highly enough. Not cheap, but you get your value for money - it's the size of a phone book and incredibly detailed.)