In all honesty, Lawrence hasn't exactly been terrible and has cleaned up several messes that the Cardinals put themselves in. He can also be used as a long man.
However, the Cardinals bullpen is not good, and a good bullpen more often than not would not have somebody with a career 6.43 ERA. While he's shown moments of brilliance, the Cardinals can't afford to find themselves in the same situation they did last year with T.J. McFarland, who was picked up off the scrap heap in 2021 and did well but couldn't replicate that success in 2022.
St. Louis needs proven arms, and Lawrence just isn't that type of pitcher.