Why Fairphone OS?

I would guess licensing issues, and the amount of work required to port.

There’s actually quite a lot of topics already covering a number of the issues you raise. The main ones are listed below (see those topics for links to similar discussions, or use the search function - the magnifying glass towards the top right of the page):

Also, for an in-depth list of features in CyanogenMod that are not in FPOS:

You may also be interested in this poll on OS preferences (which is admittedly wider than what you’re asking):

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