Belts have two advantages over chains (lower noise/vibration and less maintenance) and one disadvantage (lower drive efficiency, especially under higher torque loads).

On bicycles, the noise/vibration of chain drives is not an issue--as it is for motorcycles.
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That leaves you balancing the advantage of lower maintenance over the disadvantage of lower drive efficiency. The only segment of the market that might be interested in that combination would be very casual comfort-bike cruisers, but those bikes are typically near the lower-end of the price range for all bicycles. Using a belt would rule out an external-gear hub, and internal-gear hubs cost considerably more than externals.

So I would suspect that the only sort of people (very casual short-distance riders) who would be interested in a belt-drive bicycle wouldn't be willing to pay what an internal-gear-hub bike would cost.
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