#3 The New Day
Most of the WWE Universe love the New Day. They are one of the most comedic factions in WWE. However, that might be a solid reason for WWE to not put the WWE title on Kofi Kingston.
When talking about the New Day, we are facing two issues here. Firstly, New Day's gimmick does not fit well with a championship as prestigious as the WWE title. The WWE title is supposed to be the biggest title of the industry. WWE may find that having a comedic faction holding onto it may be detrimental to the belt.
Secondly, maybe Vince McMahon does not prefer having one member from a faction of equal members hold on to a more prestigious title. For example, in the early days of the Shield, when all three members were "equal", Dean Ambrose was the United States Champion whereas Seth Rollins and Roman Reigns were the tag team champions - both secondary titles.