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If all manufacturers used the same mounts for their cameras then they could share lenses and focus more on improving optics, sensor technology and accessories instead of having completely separate lines of lenses.
So, why are lens mounts different among different manifacturers, especially the smaller ones?
First of all, the idea that manufacturers have the incentive to facilitate the sharing of lenses is not exactly economically sound from the manufacturer point-of-view: they would rather hope that one is locked in a given system (obviously customers hope for the opposite).
Second, different mounts arise from history: there are various tradeoff to be made, different technical choices and the need to remain more or less backward compatible with one own lineup (so that you can mount any EF lens on an EOS camera, even if it was designed 30 years ago: at the same time note that, e.g., older Canon lenses do not work on EOS cameras because in 1987 a new, backward incompatible lens mount, was designed). I mention Canon because it's the one that I know better, but take thas as an example only.
As a final note, some manifacturer actually share lens mounts (witness M42 for example) or is it possible to use adapter rings to achieve compatibility to some degree (to mount a M42 lens on a Canon camera, for instance, maybe losing the autofocus capability).
PS: for an interesting take on the M42 mount, read this answer.
Why do camera manufacturers use different lens mounts?
If all manufacturers used the same mounts for their cameras then they could share lenses and focus more on improving optics, sensor technology and accessories instead of having completely separate lines of lenses.
(1) Because - if all manufacturers used the same mounts for their cameras then they could share lenses and focus more on improving optics, sensor technology and accessories instead of having completely separate lines of lenses :-)
That's a pretty convincing reason to differentiate from a commercial perspective :-).
(2) Also, users who decided that the new Nikon/Sony/Canon/xxx was better could but a new body of choice and take all their lenses etc with them. As body cost is small relative to lens collection cost the current loyalty factor would not apply.
(3) A camera of any brand could be used as a backup camera and share the same lenses. Again, doesn't help promote brand loyalty.
Lens/body lock-in is largely secondary, I believe. Different companies value different technologies or methods and to achieve their goals they've designed different mounts, different controls, etc.
Would you want to be saddled with the downsides of someone else's design? Companies are looking to push the design forward (in whichever way they feel is important at the time), and using another design limits what you can do.
Imagine if Ford, BMW, Honda all had to use the same transmission design - obviously it might work for Honda, but then BMW might not be able to make engines with lots of torque because the transmission is the weak link. Or maybe the length of the transmission makes it impossible for Honda to make a compact car, etc.
