Red tape. Medical devices need to (depending on type and country) conform to medical device regulations, be approved by a medical device regulator, be sold only to medical professionals or people with a script from a medical professional. Liability in case something goes wrong because of your device is expensive, adversarial and dependent on crossing all the 'i's and dotting all the 't's. There are "lower levels" of this due to regulations being less for more harmless devices. E.g. you can nowadays get thermometer, pulse-oximeter or a blood pressure meter quite cheaply. But anything just mimimally more complicated or critical gets expensive very fast.

Reasons for this beside the red tape are imho the low number of customers (most slightly specialized medical devices are needed once per patient with $rare_disease, once per lab or once per doctors office), the high need for customization (one-size-fits-all doesn't even work for blood pressure cuffs, let alone prosthetics), localization (broken i18n can kill, most customers are elderly and therefore not as versed in engrish), higher component cost (sterilizable plastics are more expensive, bigger displays for vision-impaired elderly clients are more expensive) and acceptance of foreign/small/unknown manufacturers (won't trust my elderly mother's health to a device from "Corty's Refurbished Asbestos Plates, Health Equipment and Luxuries Ltd., Templestreet, HongKong (CRAPHEALLTH)").

There is also a cartel of each medical professionals, manufacturers, insurance companies/public insurance pools and politicians, complete with revolving doors, kickbacks, fake or real -but always suspiciously convenient- scientific data, and exclusionary legal situations. All cementing the status quo and the wealth and standing of all participants (except the patients' of course).