Each of the makers is a little different in their policies, and long-term this is likely to have some impact on availability of third-party lenses for various mounts. Obviously, though, quantity of cameras sold also has an impact, as a third party maker needs a solid user base before they will commit.
- Canon EOS M is a closed system. Canon has not provide mount information to third-party makers. Given past history, it’s unlikely Canon will share mount information in the future, either. The Canon EF mount was reverse engineered by third parties, not licensed.
- Fujifilm’s X mount is a semi-open system. Fujifilm early on worked with a few companies, such as Zeiss, to get additional lenses available for their mount. Lately it seems that such alliances are getting rare at Fujifilm as Fujifilm themselves broaden their lens line.
- Micro 4/3 is a semi-open system. Companies can join the alliance and get the mount information. It's unclear what they have to agree to in order to get this information and provide officially sanctioned products, but we have many that have done so, with even more claiming to be committed. Micro 4/3 is also unique in that it has two camera makers committed to it, which increases the overall market for such lenses. It's not surprising that we've seen the most activity in this mount so far.
- Nikon CX is a closed system. Nikon has not provided mount information to third-party makers. Given past history, it's unlikely Nikon will share mount information in the future, either. The Nikon F mount was reverse engineered by third parties, not licensed.
- Samsung NX is a closed system. Samsung has not provided mount information to third-party makers that we know of, though there is a chance that they've cooperated with local Korean companies (e.g. Samyang).
- Sony E and FE mounts are an open system. Sony provides mount information to third-party makers who are interested in it.
Note that some third party lenses that have appeared are just mount adaptions of existing lenses, expecially the manual focus ones. To get optimal size and performance, you really want a lens that was designed specifically to the cameras that it is intended for.