In short: it needed to be capable of doing a once around polar orbit launch, which meant that when it came around back on the next orbit it would be several hun

In short: it needed to be capable of doing a once around polar orbit launch, which meant that when it came around back on the next orbit it would be several hundred miles away from the landing strip in Vandenberg. So it needed a huge cross range flight capability, which meant very large wings. Large wings meant more weight and a stronger airframe (also more weight), as well as a more complex and difficult thermal protection system. It raised the cost and complexity of the vehicle a great deal, though it wasn't the only reason why the Shuttle didn't meet its design goals.