The atmosphere is thick as soup, and that's why lift and flight are possible. Conventional rockets are way too heavy to fly (in the literal sense); they just bring tons of fuel and ditch the containers in stages.
SSTO instead brings little enough fuel (weight) that it can fly itself through the soupy part, which is much more efficient (by like an order of magnitude), on top of not requiring oxidizer for that phase. The longer you can stretch out the aerodynamic phase (i.e. the faster and higher you engines breathe), the less oxidizer you need to bring.
If you crunch the numbers (which apparently they did), you get that the equations work out to needing an engine that will stretch the aerodynamic phase up to mach 5. If your plane can't do that, getting to orbital velocity requires you to bring too much fuel for your plane to fly in the first place, and you might as well build a conventional rocket.
SSTO instead brings little enough fuel (weight) that it can fly itself through the soupy part, which is much more efficient (by like an order of magnitude), on top of not requiring oxidizer for that phase. The longer you can stretch out the aerodynamic phase (i.e. the faster and higher you engines breathe), the less oxidizer you need to bring.
If you crunch the numbers (which apparently they did), you get that the equations work out to needing an engine that will stretch the aerodynamic phase up to mach 5. If your plane can't do that, getting to orbital velocity requires you to bring too much fuel for your plane to fly in the first place, and you might as well build a conventional rocket.