The reason is that Mint was created for an exit like this. They had no clear business model other than getting as many users as possible in order to scare Intuit. They have a great product, but I see no plan to make money with it.

Of course they have the thing of suggesting credit cards to users, but it doesn't seem that they were making any money on that...

In summary, if Mint was profitable, they wouldn't sell the company in the first opportunity.