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.
They do reprint it periodically—seems like it gets a bunch of specialty versions. But it hasn't been printed in a standard set in almost 15 years, as far as I know.
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.