Granting a pardon doesn't mean admission of guilt, especially for the Deep State Conspiracy theorists -- it would only mean admitting "the conspiracy might falsify evidence and pull the wool over the eyes of a judge and jury, so I'm avoiding the problem by preemptively pardoning."

I expect what happened is, every lawyer he ran the idea past said, "err.... no, it won't hold up. Even in your very conservative and grateful-to-you SCOTUS, the courts are not going to allow that presidents can just pardon themselves of everything on their way out of office. Nobody wants that precedent."