CreditKarma was far from privacy focused, their T&C + Privacy policy gave them lots of grounds to mine and sell user data.

Personal income information about users is generally 3rd party estimates for ads/marketing industry, but by offering a free Tax service, CK was able gather information on actual income about users, which is extremely valuable data.

This would be very lucrative for data-brokers or sold as targetable and proprietary 'audience data', which can be combined with other data-sources to support marketing activities.