I worked in medical tech for a few years, there are a few reasons people aren't going to flood into that sector.
1.) Regulation, HIPAA is a massive pain to deal with, the rules nebulous and require significant process that is expensive and time consuming.
2.) Violations of HIPAA lead to fines and even lawsuits from the office of civil rights. It used to be that Business Associates could hide behind the medical institutions to avoid this but with the Omnibus rules you'll face massive fines that will easily bankrupt any startup.
3.) The entire healthcare sector is extremely risk averse, slow moving and you cannot work around them you have to work with them which means you're going to move slowly too.
4.) You will HAVE to integrate with dozens of other ancient applications that medical institutions use, some of which have no documentation or publicly available source code. I hope you like searching through hundreds of pages of outdated HL7 and X12 documentation to figure out how a specific vendor screwed up their implementation.
5.) Large medtech companies regularly snuff out competition through lobbying and leveraging existing relationships in the medical community. So you've got a killer new medical app that will change the world? Big deal, your competition has three doctors who are leaders in their field(and conveniently board members). They will tell all of their buddies at the next American Cardiology Conference that your app is garbage. I hope you've got some big names associated with your startup and you can pay them handsomely.
6.) The medical field is a data hell. Some of it unstructured and non-sensical, much of it is structured but has no validation. That field you are getting from a third party API for albumin levels which was documented as g/dL, was actually entered in mg/L from the years 06/2004-12/2008, mg/dL from 09/2003-05/2004 and the proper documented g/dL for all other dates. Why was it that way? No one knows. You have to keep track of things like that for one field in one database for a single department in a medical institution with a few dozen departments that don't cooperate. Worst of all, it WILL change on you without you knowing.