To get to an appropriate speed, you need to accelerate eventually. But it's easier to do that at a higher altitude, where you have thinner air to travel through: less drag, less severe transonic effects to mitigate and, on the way up, a wider variety of air-breathing engine types to choose from (including the Astro Mechanica one).
The wider variety of air-breathing engines points out a problem: the launcher necessarily goes through a wide variety of aerodynamic regimes where different engines work. Making a single engine that works over a wide range of speeds is difficult. And for what? Using more fuel in a larger first stage just so you can save on cheap oxidizer (LOX at $.10/lb)?
