Why is the historical present tense used so often by sports broadcasters?

Keeping away from language in general for now, I can state that the use of the present tense makes things feel more immediate to English-language listeners. Thus it is used to create a sense of action and dynamism. Very often when telling a story, especially on a theme like sports, in which a certain dynamic tension is essential for effectively conveying the teller's experience, storytellers use the historic present for this reason. Often this tense is used without deliberation, instinctively, to gain impact.

Spanish, and I suspect other romance languages as well, make heavy use of the historical present for narrative as well.

It is an example of the erosion of the conditional in popular speech. The reason probably has to do with the fact that a true conditional statement just takes too many words for a sports announcer to say:

If he would have caught that ball, he would certainly have gotten into the end zone.

That (together with variations like "Had he caught ..." and so on) also probably sounds fussy and over-particular to an audience of more or less intoxicated sports fans. Much easier to say

If he catches that ball, he's in the end zone.

It's simple, direct, and has a manly feel to it, making it perfect for sports. And as the language of sports is popular and omnipresent, it becomes a big driver for the language in general. Don't look for it to go away anytime soon.