The OP probably means "solved" as in "faster."

The original Conda solver is just badly optimized, whether through the implementation or a poor language choice. For years, user performance concerns were written off as just the price you pay for the correctness of an SAT.

Mamba was a third party rewrite that was a couple orders of magnitude faster. Users started leaving Conda for Mamba. It scared the Conda developer enough that they dropped whatever invisible back-end features they were prioritizing, and within months they added a feature to install different dependency resolution libraries, targeting Mamba first.