1) High touch sales process, slow sales cycles - ultrasounds take a long time to sell to hospitals/clinics so the sales process is very high touch and requires a significant number of demos and in person time. Those sales reps are usually paid on commission. Cheap products don't make sense to sell this way.
2) Lack of competition and high development cost - Ultrasound machines are a Class II medical device and so are regulated by the FDA. FDA requires compliance with most relevant ISO standards, and there would probably be a dozen relevant standards for an ultrasound machine.
3) Liability - complex medical devices that are used for critical diagnoses create big liability problems for the hospitals/clinicians. They have no incentive to choose the low-cost model sold by a startup. No one ever got sued for choosing GE/Philips/etc.
4) Lack of price transparency - its often hard to find prices for big ticket medical devices, so the natural pressure to reduce costs through competition isn't as effective, especially with a high touch/high cost sales process.