R.J.
Because kids hearts look different than adult ones do.
Pediatric Cardiologists look at children's hearts all day. My son's heart was 'very concerning' to adult docs, and 'Oh. This is totally normal. Beautiful. Fantastic.' via the Children's Hospital Cardio team.
Conversely, my friend went through hell with her son (in and out and in and out) of their local hospital until they FINALLY got a referral to Children's. Their local specialists said he was perfectly fine. Children's spotted the problem in (quite literally) a heartbeat.
I live right next to a Children's hospital, so I have it easy. When my son was sick last year from Feb-Jun, we had the BEST care in the PNW, and some of the best in the country. Sooooo many parents I met there, though, had spent months and even years with misdiagnosis, waved off concerns, wrong treatments, etc... because local hospitals rarely deal with kids. Things that a children's hospital sees a hundred times a month, the pediatric cardiologists/pulmonologists/neurologists/etc. at a local hospital MIGHT see it once every 10 years. Or not since medschool.