I'll explain.
Basically, the Cowboys felt as though Buehler could do better than a touchback. When he was kicking touchbacks, you will notice he kicked to the right.
Joe D felt like if Buehler angles his kick to the left and put more under it to the 1... we would gain more by stopping them inside the 20 (like his last kickoff... you'll notice it was very left to the corner and we stopped them short).
The reason why he's not kicking touchbacks is because they don't want him to. It's been discussed a lot by the crew of the Lunch Break. They feel like the opportunity to pin them inside the 20 is worth the risk of something as rare as a kick off return TD.
I dunno how well I explained that, but that's the jist of it. If he kicks to the right, he's going for a touchback usually, if its more to the left, he's trying to kick high to the 1.