Probably because accessory/detail part manufacturers tend to follow a new kit with parts for it, which can be around for sometime, or withdrawn, then re-released when or if the kit is re-released. In the case of the Revell kit, it was first issued when most of the people running the accessory companies weren't even born! In other words, it's been 'replaced' in the market by newer offerings, which is not to say it's a bad kit, probably just as good as some of todays kits, although with raised panel lines and maybe not some of the finesse of modern moulding techniques.