It has items, locations, Reckless challengers, Huey-Dewey-Lewey combos, and a couple of Rise of the Floodborn cards that weren't playable back then and still aren't playable now
5 Sorcerous Squall Free Spells For Three Mana (Kinda) Delve is an interesting mechanic, one that is very, very good at letting you cast big spells for much cheaper
Cards which slot into already existing archetypes Ignoble Hierarch in Jund Ignoble Hierarch gives the Jund colors access to a mana dork of the same caliber as Noble Hierarch, and any deck from BG Infect to Goblins will be looking to try the card out
There are players who want it and like it
While not the most valuable Fossil card, the fiery Kanto Bird has become a favorite from the 1999 expansion
The inclusion of Treasure Mage in your Type 4 stack should be directly related to how many big artifacts it can fetch