Like many things when it comes to designing hardware for photography, there are always tradeoffs to be considered and made.
STM lenses sacrifice a little speed in order to be quieter and smoother (no jerky starts and stops). This is important when using Autofocus while recording video.
Lenses with USM focus designs are built for speed first and quiet operation second. Since they are optimized for still images, the jerky starts and stops that help them get there faster are of no consequence.
More advanced videographers tend to use manual focus, often with external focusing rigs that attach to the lens and allow much finer manual focusing control, and also enjoy the benefits of the superior optics in many "L" series lenses. That is, if they are not using the even more expensive Cinematic grade lenses that in addition to superior optical image quality include such features as parfocal zooming and corrections to eliminate focus breathing (but no AF).