There have been open and de-facto open mounts though: the Pentax K-mount was such pre-autofocus, and micro 43, Sony's E-mount (albeit with many strings attached), and the L-mount all have varying degrees of openness about them.

At the end of the day, though, camera owners don't prioritise that enough in their purchasing decisions to change the market. People were happy to lock into highly proprietary mounts when Minolta developed autofocus (and migrated to the A-mount) and then Canon followed with their EF.