1) pass blocking is much harder than run blocking and requires a completely different mindset (you cannot attack in pass blocking or you're going to get beat). The DL has a huge advantage on second and long because they know a pass is coming and they can stunt/blitz and that makes it even harder. When we throw an incompletion on first down the defense knows to prepare for two more pass plays and the DC can send exotic blitz looks.
2) When you call more running plays you can wear down the defensive lineman/linebackers because they are smaller than the OL and fighting off those blocks will tire them out. However, they are much better athletes than the OL so if you don't call those running plays you wear out your OL because now they have to move much more and don't attack in short spaces.
3) zone running plays are difficult for young OL to grasp and that appears to basically be the only types of running plays we call.