#4 Because another loss would have derailed Kingston's momentum
If you actually think about it, Kofi Kingston has not been winning a lot really. After all, he came up short in the Elimination Chamber against Daniel Bryan. And let's be honest, while he had an Iron Man Performance in the gauntlet match, he came up short against Styles in that particular match which was won by Randy Orton.
So, having Kofi Kingston go up against Daniel Bryan and losing on the stage would have killed his momentum. Conservely, having him win at Fastlane would have just robbed him of a WrestleMania moment. By booking the match at WWE Fastlane as they have done, it plays out in the best manner possible for all the parties concerned.
If Kofi Kingston suffered another loss, maybe the fan support would have waned as well. Nobody likes to support a loser, all said and done.