[bspn]

In short, liability. So much behavior within the US medical profession is driven by a fear of getting sued for malpractice. Even though the patient would be sitting in the ER with treatment close at hand, if the situation did worsen and the patient suffered complications there will be the inevitable questions asked as to why the triage nurse didn't admit them in the first instance. For hospitals it's just not worth the risk of potentially expensive and lengthy lawsuits/settlements.