It's because our politicians are largely owned by our corporations and spend a ridiculous amount of money protecting their interests [1]. We almost had a public option with the original "Obamacare", but it was forced out of the bill [2].
Also, just turn on Fox News for an evening and realize it's been the number one news channel in the US for 20-something years. They've been a right wing corporate propaganda machine for a long time, all while brilliantly portraying themselves as the "underdog" fighting the mainstream media. Americans aren't very educated and take pride in their ignorance, unfortunately. [3]
[1] https://en.wikipedia.org/wiki/Citizens_United_v._FEC
[2] https://en.wikipedia.org/wiki/Public_health_insurance_option
