GP's being capable enough isn't the issue. It's much like blood draws.

GP's are more than capable of drawing blood an sending it off to a lab, but that process requires significant support of outsides parties. It simply doesn't make sense for the GP to take care of it in house when they'll likely need to send it out to someone else.

In the care of ultrasound, it also introduces an issue of "certifying" results. Taking an ultrasound isn't hard, but reading it can be difficult. Even if a GP can read an ultrasound with 90% accuracy, it's still much lower than someone who specializes in reading digital imagery. Of the 90% that a GP can read accurately, how many of those can a GP read confidently? Certainly not 90%. At that point, they might as well send you to someone who will get it right the first time.

http://www.physicianspractice.com/blog/three-good-reasons-of...

(I say providers above because doctors are often just part of the machine, but the machine doesn't want to waste the doctor's valuable time any more than an independent doctor does)