The colour stabilisers were very expensive, so many mainstream manufacturers never bothered putting them into the paint due to the cost. The Vauxhalls, Nissans and VW's, all the everyday brands, etc, would fade, but the Jags, BMW's and Bentley's generally did not. Modern acryillic-urethane car paint compounds don't suffer in the same way,
I would imagine the price made it completely prohibitive for bicycle paint back in the day. As the team above have rightly pointed out, it probably then spent a period stored in a shed or building where the sun caught it daily, and nature took its course. On the plus side of the ledger, thats a good excuse to get it powder coated.