Maybe for one reason, if a maker wishes to brag about his mounts being patented, he's got to throw in some little complication that other mounts don't have.

Take that L-shaped piece inserted into the inner face of the adjusting screws.  It's purpose is to make the tube move in a straight line, rather than at an angle, as it would if the tube isn't perfectly centered in the mount. Fecker was the first to patent such a device, although before that, some Stevens mounts were fitted a round, movable tip that did the same thing, but was never patented. 

Mitchell's all though quite modern used Micrometer heads on the external mounts and the click system was actually quite simple but the mounts are quite large and not very attractive but the glass was the best. 

JLouis