The way I see the whole Twitch thing is that - on top of Halo fading out the public sphere and being "difficult to get into" - the games haven't been interesting to watch for a very long time. Out BRing people just isn't cool anymore. There aren't many personalities in the community that can sell the game either.
Halo 5 at least has potential to "please the crowd" with the Pistol start and new movement abilities, but the maps are lagging behind in that regard.
When you're watching any other game, it's either for the "trick shots" and clutch plays, or the strategy. Halo in recent years has not excelled at any of those, and it's only in the serious competitive games where that experience shines.
Console exclusive will explain it belong lower than multi plats, but not as low as it is.