What makes a particular item popular as a collectible at any given time?
Thirty years ago everyone wanted Depression glass. Now you can’t give it away. People that come to my tag sales are now asking for sports or military memorabilia, coins, comic books and old fishing lures. Why are these such hot collectibles now?
Really, it all comes down to hype or the value of the item to collectors. I know in the case of heroclix (my husband plays) or Magic The Gathering Cards, it comes down to the playability of the clix or card and the best powers or stats for the point cost. Other times it’s a limited edition "we’re only making 100" type thing. So I think it varies in the subject matter.
Simple. The fickleness of the general public.
Media hype, just like paparazzi with celebrities.
Beanie Babies were it a few years ago and Elmo. They are in short supply and that raises the demand.