Fallout is easier to translate to the masses because it's not as linear as Halo.
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Halo was a mess because it was a mess. Halo didn't need spirit quests, love interests, random political drama that derails its own source content, etc and it suffered for it. Fallout staying generally faithful to its own aesthetic stands out simply because Halo's production misunderstood the assignment. Nolan's clearly stayed attached to the IP's foundation and it pops accordingly.
Halo was so oversaturated that it was few storyboards away from being a new franchise altogether.