National Geographic photographer Bob Krist explains why he moved to Sony

[Woody]

Sony know that F/2.8 glass would be too large to balance with the A7 cameras, it might also have issues with the mount size.

Indeed.

As pointed out by Sigma CEO:

"It's a bit more difficult to make ART lenses for the Sony FE system because of the not so large diameter of the mount. We don't know why Sony did this. Likely because the E-mount was meant for APS-C first and only after that they did use it for FF too."

- http://www.mirrorlessrumors.com/sigma-interview-mirrorless-is-growing-and-we-will-make-more-lenses-for-it