Power creep. I stopped playing in 2004 or so, and the number of expansions since then has been a tad more than 20. And the number of traits and abilities has skyrocketed.
You say that, but back in the day I played red, and the quintessential red card—Lightning Bolt—is now considered too powerful to keep in print.
These through the ages cards are only reprints for formats like commander where it was already legal.