Personal experience, mostly, having drunk the julep both ways.
History comes into it in that most times I have read accounts of people drinking juleps they make a toast and drink them down. They don't sip on them for an hour. That is probably what gave me the idea. There had to be an answer, because I couldn't imagine that Kentuckians would invent such a crappy drink.
Also, when you think about the accoutrements--crushed ice, metal glass--it's clear that the idea is to chill the liquor quickly, which also leads to dilution if you don't drink it right away.