Or because one purports to measure achievement and is perceived as gameable and achievable through competitive efforts, whereas the other isn't. And thus IQ is seen as something like height which would never be used as a selection criteria...

Alternatively, I know the word on the street when I was a kid was to never tell a child their IQ as it could damage them for life. So, maybe some of that shhhh.... trickles into the same meme space as the jolly guy in the red suit.