Lou Brock has a number of things contributing to his low WAR.
First would be that despite his speed, he was never considered good enough to be a regular CF.
Playing LF his whole career comes with a -108 positional adjustment. Total Zone thinks he was -51 fielding runs to boot, further dragging down his value.
If he was an average CF, that would add about 20 WAR. But he was a below average LF.
The other main thing holding him back was lack of walks (79 BB+) and power (94 ISO+). 2,247 of his 3,023 career hits were singles.
Even his speed doesn’t add up to as much value as people think comes with being the one time stolen base king, because his success rate (75.3%) was so low compared to other great runners like Rickey (80.1%), Raines (84.7%), Coleman (82.8%), Morgan (81.0%), Lofton (79.5%), Ichiro (81.3%), etc.