Three reasons:

1. Exit polling is intimidating.
Anonymous voting frees you to vote your conscience. No one would like a gang outside a polling station ready to find out how you anonymously voted. What’s the point of anonymous voting if anyone is asking you how you voted, immediately after you did?

2. Exit polling is inaccurate.
Those declining to answer an exit poll cannot be shown to be more in favor of one candidate or the other. Yet exit polls must accurately reflect how all voted. Because of reason #1, it’s likely a percentage of exit poll results will be not be true. So it can potentially introduce a falsehood into the results. Results must be both logically and mathematically valid. Here’s an example – suppose you’re trying to solve an equation: x = 2+2. Suppose the first 2 is not really a “2” but “1”. Do you think the result “x=4” is going to be accurate?

3. Exit polling skews decisions.
Publishing exit polling results encourages a mob mentality – go along to get along. It encourages both intimidation and inaccuracies, making it possible to sway some portion of the electorate to vote based on immediate self-preservation reasons. Given the possibility it may skew the decision, it creates a feedback loop. Anyone who’s ever heard the high-pitched squeal when a live microphone comes near an amplified speaker that’s playing that input understands how a feedback loop is problematic.

At it’s least intrusive, exit polling is misleading, at it’s worst, it’s a potentially distorting device that should be eliminated.