Complex sentence: The popularity of card modeling boomed during World War II, when paper was one of the few items whose use and production was not heavily regulated, but as plastic model kits became more commonly available, interest in paper decreased.
Split into simpler sentences: The popularity of card modeling boomed during World War II, when paper was one of the few items whose use and production was not heavily regulated. But as plastic model kits became more commonly available, interest in paper decreased.