I can't understand why Canon would persist to make F4 lenses, specially ones that are designed to be upgrades (Fisheye 8-15mm f4).
1) Price. The 16-35mm + 24-70mm + 70-200mm trio at f/4 is 40% cheaper than the f/2.8 trio.
2) Weight. The f/4 trio is 33% lighter than the f/2.8 trio.
3) f/4 IS vs. f/2.8 IS-less. As explained by others, IS compensates for hand movement, aperture helps freeze the subject.
As for the fisheye zoom, I think it's a different story altogether. Canon could have gone two ways:
1. Make four fisheye prime lenses - circular & diagonal, each for APS-C & FF, like Sigma did. That means dealing with overhead of four niche lenses, all the way from design, through production, marketing, stocking, and stocking parts to fix sold lenses.
2. Make one fisheye zoom lens. It misses the niche market of circular fisheye lens for APS-C, but has (a) saves on said overhead, (b) is a lens the owner can keep when upgrading to FF.
Also, as shallow DoF is not a consideration for fisheyes, an ISO stop would usually compensate for an aperture stop. There might be night time photographers who would like f/2.8 today, but improvements in sensors might make this a moot point long before the fisheye zoom reaches end-of-life.