Why don't they just make their own? Seems like keeping that data 'in house' would be wise.
From the perspective of the investors, they are keeping it "in house" By pairing work of two investments, they make both a little stronger (well, bitly a lot stronger).
However, there's no reason to use one (and lots of reasons not to) when the URL doesn't break the 140-character limit, or when the message isn't going out via the SMS gateway.
I'm not holding my breath for Twitter to implement such logic, though; they seem to have a truly unhealthy fascination with URL shorteners.