Mark confirmed in his Tevesh Szat preview article that there's a full monocolor mythic partner cycle, their Wills, so maybe they're just holding it back. Plus, Commander Legends has 361 cards in the main set alone, so mythic numbers are way up (even if you ignore the foil etched-only cards).
The legendary creatures are also on their own sheet, whenever they do something like that they have more wiggle room with cards (such as Innistrad with the DFC)
Yep. Specifically, there are 22 mythics (5 of them legends) in this set, so we're looking at around 3 non-legend mythics for each color. Since there's already 4 artifact/land mythics, at least some colors won't be getting the full 4 mythics.