It would take billions of dollars and many years to build a new general purpose EHR from scratch. This isn't an industry where a start-up can launch an MVP and get sales to early adopters looking for a competitive advantage. Every major provider organization already has an EHR, and usually an ONC certified one. A new offering has to be at least as good as existing products in every way in order to get any sales. It has to be a 100% solution out of the gate.
Outside of general purpose comprehensive EHRs there is still opportunity for new niche market offerings targeting limited medical specialties or types of facilities. For example if you wanted to target, let's say, just dialysis centers or just psychiatrists then the hurdle would be much lower.