Why could one vague Backrooms photo grow more lore than a finished monster story?

Answer: Readers can fill the gaps

Readers can fill the gaps ✓ — Right. Creepypasta spreads well when it offers a believable frame and leaves room for users to add levels, rules, encounters, and proofs. Underberg-Goode describes creepypasta as changed, adapted, revised, and expanded across media. The Backrooms began as an image plus a short premise, which made it less like a closed story and more like an invitation.

Official canon fixes rules — No. Fixed official canon can help a franchise stay consistent, but it can also reduce participation. Early Backrooms lore grew because users could extend the space, not because a studio had already locked every rule. The open edges were the engine.

Short posts spread faster — Plausible, because short posts are easy to copy and pass around. But brevity alone does not explain why people kept adding to it. The useful feature was open-endedness: a place to get lost in invites new levels, exits, entities, and fake evidence.