Keruga, the Macrosage This is probably the easiest card to evaluate since it has an obvious use
Leyline of Anticipation and Vedalken Orrery set us up for success, while Vedalken Mastermind, Erratic Portal and my personal favorite Crystal Shard really knock things out of the park
But if you have Capricious Sliver on the field, you'll never run out of gas
Niv-Mizzet, Parun is the textbook commander for this style: you control the early turns with counters and removal, then use those spells to protect Niv during a Curiosity combo finish
The answer is probably relatively simple: once the allure of Mega Hyper Rare cards wore off, collectors simply preferred the arguably superior artwork of an SIR
Stamped With Partial/Missing Foil: Visually similar to missing stamps (well a little bit similar at least) cards sometimes get stamped without all of the foil