The short answer to it might just be that's it's a difficult pitch to execute? Tunneling approach as a pitcher would obviously tell you to do that, and I'm sure pitchers are trying. But with a sinker, you're running the risk that it comes back into the heart of the plate as a very hittable pitch, and a hitter like Vlad is pretty adept and taking an outside fastball and sending it the other way for a hit.
It's kind of like how backdoor breaking balls are usually a decent pitch, but if you miss with it it's either an noncompetitive ball or a hanging pitch. Maybe pitchers just don't have the confidence to execute those kinds of pitches, consistently.