Have you ever had a good look at Stipe's body (no homo)? That is a guy not built for head kicks. Long torso and relatively short legs; in fact he's got the shortest leg reach in the HW division. That's right, fireplugs like Hunt and DC outdo him in that category. In fact, before he had back issues and started stretching exercises to mitigate it, he wasn't even limber enough to kick high!
EDIT That being said...He did try a head kick in the Overeem fight, though it's easy to forget that since the next notable thing that happened was the Reem decking him. And I did lately read of someone -— I think one of the coaches he had on TUF — praising his high kicks, so maybe he's been working on them since the DC fight was booked?