[InclinedPlane]

The main reason is merely tradition (really). Most systems are designed as evolutions of older systems, if the first system used vertical assembly, then so will the latest.

Note that the Russians have been doing horizontal to vertical launch vehicle assembly for decades (with manned and unmanned launches). There's no engineering reason why you must prefer one method or the other, it's a choice.

http://www.globalsecurity.org/space/world/russia/images/usta...