Why not M4/3?

I wonder why basically only Panasonic and Olympus, still, have joined Micro Four Thirds. Why not Sigma, Pentax, Sony, and Fuji (I don't even include Nikon and Canon, they are ultra-separatist)? Is it a kind of ego thing, they have to "be their own man" as a corporation? If they joined M4/3, they could immediately sell camera bodies to people who have the lenses. And if they make good lenses as time goes on, these may be bought by people who have bodies from other manufacturers.

For example, the otherwise very interesting Fuji X1-Pro (or maybe X-Pro1, sigh*.) is hampered by only having three lenses so far, and no zooms. It takes tonnes of money and time to develop a good lens line. Why not join up to an already strong lens line? And the sensor size is almost the same anyway. Why all this super-pride.


*As said in a review of the new Canon G1X: 

Just a few months ago when I reviewed the Panasonic GX1 I joked about the number of new cameras with X in their names. This then included the Leica X1, Canon 1DX, Samsung NX200, Fujifilm X-10, Ricoh GXR, Casio EX15, Olympus XZ1, Sigma DP2X, Sony HX9V – and that's with just one model from each company. Since then, in addition to the Panasonic GX1 and the new Panasonic X series lenses we have had the Fuji X1 Pro and now the Canon G1X. 
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