...you gotta love this article. It explains things quite well. I especially like this paragraph:
In other words, the baseball umpire enjoys a prominence above all other sports officials in that his authority is essentially perceived to be beyond question. Any use of instant replay would diminish this elevated status. It would also reduce the paying customers' need to yell, "Kill the umpire." After all, no one bothers to abuse NFL refs anymore because most of their power has been technologically neutered.
Read the whole thing.