The question isn't so much why Intuit would buy Mint, but why would Mint allow themselves to be acquired so soon? If Mint was doing so well, they'd continue on their path to completely outplace Quicken. Something about this just doesn't add up.
VCs don't fund startups to achieve competitive victory per se. Their job is to make money for the people who invested in their funds, and they do that by selling the shares they bought for more than they paid for them. The main routes for that are acquisition and IPO.
Right now, the IPO market is not looking healthy, and won't be for a while. Given that Mint's revenue comes from getting consumers to sign up for credit cards and other financial offers, their market will be anemic for a while. So they're an especially poor IPO candidate for the next few years.
That leaves getting bought. If that's what your after, and Intuit comes along and offers you a great price, why wouldn't you take it? Intuit needs Mint a lot right now, but maybe later Mint's bargaining position will be worse.