It turns out to be very hard to achieve orbit with a winged vehicle that takes off like an airplane, because it's very difficult to achieve a high mass ratio with a craft that has to have heavy wings, landing gear, and air breathing engines. It's certainly not impossible, but it takes a whole lot of development and many projects get canceled when the numbers indicate they won't deliver a useful payload, if they can even achieve orbit. For example, the MUSTARD proposal's lift-off weight is nearly as much as the Falcon 9 rocket but it was only planned to deliver about a fifth as much payload to orbit. Yet the vehicle acted like a conventional two-stage rocket with flyback capability. Given the estimates of the Skylon's development and operational costs and payload given to Parliament, if its launches are priced competitively (in $/kg) to the existing SpaceX Falcon 9, it would have to fly 411 missions before it hits break-even, and that's ignoring the time-value of money and assuming no cost overruns or accidents. If the SpaceX Falcon 9R succeeds and lowers their launch costs, the Skylon might not ever be competitive. One of the issues such vehicles face is simple launch weight. No supersonic aircraft has ever been built that has a gross lift-off weight of more than about 600,000 pounds, half the lift-off weight of even a medium rocket. And of the large supersonic aircraft (Concorde, B-70, B-1, and Soviet designs) none has carried a fuel fraction of more than about 40%. This can no doubt be improved on, but no such vehicle (mere supersonic, not even suborbital) has been remotely cheap to develop or operate.