What's the rationale here? Nobody can really DO anything about votes anyway, so why limit the ability to see how the sum is reached? In a few SE sites, such as this one, I'm rather low rep and can't see whether something's been voted up 5 times, or 6 times and a -1, etc... I've found it frustrating and am having trouble thinking of why it would be limited to begin with.
Seems like similar to - Show Total Votes (or Up/Down Votes)
Taken from that link - As quoted by Jeff Atwood -
The total vote count (score) is denormalized, but the individual up/down vote counts are not.
So to display it on every post would incur 2 vote table queries * number of visible questions / answers. Our DB is fast, but the vote table is pretty massive, and not doing a query is always faster than doing it..
Because it's mildly interesting? You find it interesting to see those stats, therefore setting a specific reputation level gives you the incentive to gain reputation to see those stats.
Edit: Plus, it looks like there was a "desert" for privileges at the 1000 rep level, so this filled that gap nicely. It's since been filled in with the chat room priv, but that's a relatively recent addition.