To go a little deeper than this, in north Mississippi and West Tennessee he was the right man at the right place with the right troops at the right time.
It was a back water section of the war staffed variously with a scattering of Federal infantry, garrison troops, and cavalry. The Federal cavalry was fairly substandard, and didn't really have any sense of how to utilize their cavalry in a combined arms body beyond scouting and screening. Forrest's troopers were largely mounted infantry, which had a decided advantage over the Federal cavalry prior to the dissemination of repeating carbines. In Forrest's greatest victories, he would wait for the cavalry to arrive, attack, force the infantry to make a forced march to reach them, and defeat them both in detail. The infantry, exhausted from the forced march and disrupted by the panicked cavalry, made east targets.
In larger set battles of a more conventional nature, Forrest was mediocre at best.