Everything uses HL7, every last bit of equipment that has patient details on it, and people are familiar with working with it. This will be why they do it I think. I don’t think it can change now as it’s so deeply entrenched.

We're certainly starting to see a gradual transition over to XML/JSON via FHIR, and I've even worked with a couple of vendors that are pushing RESTful web services as their preferred integration method, but agreed: there's absolutely massive inertia behind legacy HL7.

Implementation costs for a primary EMR are in the millions and most ancillary systems still cost hundreds of thousands of dollars to put in place. There's a lot of financial incentive for healthcare organizations to keep using what "works", even if it is ugly as hell.

The SEG|1|Field^^^|| etc stuff isn't "HL7", it's a specific HL7 spec, V2.

HL7 FHIR here is JSON and XML based.