POCUS is used commonly in my experience and there are large amounts of curriculum devoted to it. We haven't had affordable access to POCUS for decades so I don't see how doctors have refused to learn it for decades.
Ultrasound has limited penetration for deeper tissues so you aren't going to see lung cancer with it (especially behind ribs), and overweight patients don't image as well, again because of limitations in imaging depth. Regardless of uptake it's not going to solve all our diagnostic challenges.
It also takes skill to know what you are seeing and to know how to find what you're looking for. Anyone could learn, but it's a time investment and physicians have limited time as is.
Still, it is becoming more popular and soon it won't be uncommon for your primary care doc to pull out the probe and see if your gallbladder is inflammed or if that abscess is going to be amenable to in office drainage.