Americans love underdogs because they care a hell of a lot more about their feelings than being correct. When an underdog wins, it feels way more powerful than when the big guy takes yet another title- that just isn't exciting, it's expected. Look at any sports movie ever- they're all about underdogs. Recently, particularly among the youth, there has been more of a shift in backing the top dog (take, for instance, our recent love of Tony Stark and Bruce Wayne at the box office- but in a way, even Tony Stark is an underdog, just a guy in a tin can fighting demigods), but on the whole, America has always been all about rags-to-riches or nobody-to-champion stories. They hit is right in the feels, everytime. There's a lot of good papers out there about it on the internet, as it's a very interesting (and very American) sociological phenomenon.
That's why nurses are where they are today. People know that nurses aren't as smart as doctors, that they aren't as qualified, and that they probably don't provide the same level of care. But there's something about the idea of a nurse that's almost as good as the top dog doc, by virtue of hard work at the bedside, rather than a long time in school. Something that makes them say, "you know, maybe that nurse is just as good, maybe they just want to be free of the shackles of the man." And then they're so personable to boot, who wouldn't want someone that gives them the warm fuzzies rather than the clinical once-over as their caregiver. They will literally risk their lives on independent care that hasn't been thoroughly studied because nurses have good PR and the idea that somebody from the middle class like them, rather than some ivory tower doc, can provide them with care. Don't ever underestimate just how much logic an American will throw under the bus for a good narrative and some feelings. Logic will always take a back seat.