I Am One With The Force
The Force Is With Me
One possibility is that while artifacts are popular, they're also really powerful. Thanos and Apocalypse in particular go down much easier (though not always easily) when artifacts are involved.
The designers might have decided to shy away from the concept in order to prevent power creep. At least one poster here is worried this game will go the way of the Overpower card game, where new characters are almost always more powerful.
Good point. Makes sense why they transitioned to Thrown artifact in Fear Itself.