For the case of 35mm SLRs, Nikon/Canon/Pentax etc. all have 45mm flange distances, give or take a couple of millimeters. That's basically the minimum to have ro

For the case of 35mm SLRs, Nikon/Canon/Pentax etc. all have 45mm flange distances, give or take a couple of millimeters. That's basically the minimum to have room to swing the mirror and for a couple of shutter curtains. So all of them having completely incompatible mounts is because they wanted to compete based on lock-in to a complete system, rather than compete for bodies and lenses separately. Even worse, the fact that their flange distances are all within a couple of mm makes passive adapters impossible, since just the bayonet part of the mount is longer than that.