Slightly different requirements.. X-15 was single seat, no provision for sight seeing, trained test pilots that could handle high G forces, etc..
Also, X-15 had much more tolerance for failure, frankly. When you're doing something for the first time with military test pilots, you can be 95% reliable. Virgin has to be nearly 100%, and that last 5% is a bitch.
Also, X-15 had much more tolerance for failure, frankly. When you're doing something for the first time with military test pilots, you can be 95% reliable. Virgin has to be nearly 100%, and that last 5% is a bitch.