Because he genuinely has a somewhat idiosyncratic style of legal reasoning even though ideologically he's a run-of-the-mill conservative hack.
He's not run-of-the-mill. He's the only justice who has called for Batson v. Kentucky and NY Times v. Sullivan to be overruled. He's the only justice who has ever compared Planned Parenthood to the Nazis. He's the only justice who has ever cited James O'Keefe.
I know what you mean, and Thomas is one of my least favorite people in American public life, but in fairness to him, presumably the two justices who dissented in
Batson (the incumbent and immediate future Chief Justice, no less) also felt that it ought to be (if possible) overruled.