Because in their minds the batter seemed to care more about his average than helping the team. Here's why. With a runner on first base and no outs, the first baseman will be holding the runner at first which creates a bigger hole on the right side of the field. If a hitter can be patient and wait for a pitch on the outer half of the plate, he can try to hit that hole with a ground ball or line drive. Not only does it help his average but it helps his team more as well. If the ball shoots through the right side, the runner on first base will have a much easier time going from 1st base to 3rd base on the play.
Having to hold a runner on creates the hole at first
for the hitter to shoot the ball through.
In the previous example, the batter did not attempt to hit that hole or "hit behind the runner." He swung at the first good pitch and hit it to left field. Although he did get a hit, the runner on first base will have virtually no chance to go to third base on the play because of the much shorter throw from left field to 3rd base as opposed to a throw from right field to 3rd base.
Of course, like most other hitting situations, what is expected of the hitter often depends on who the hitter is. If the leadoff or 2nd hitter in a line-up is batting, hitting behind the runner makes a lot of sense. However, many coaches will not want their best hitters (usually the 3rd, 4th, and/or 5th hitters) to do anything but swing away as normal. They are the RBI guys so many managers will not want them just to try and shoot a ball through the right side.
I think this makes sense to most old timers. What they can't stand is the increasing number of non-3-4-5 hitters who make no attempt at all to hit behind the runner with no outs.
Hitting behind a runner is a difficult skill for any right handed hitter. It starts with a willingness to try it and then lots of hard work in practice to master the art of doing it.
Tomorrow: Part 2: How to hit behind the runner!