Decent people have compassion for the poor, sick, and less fortunate, even if they've been lucky and never had health problems or had to struggle financially. We can read about people who are struggling without necessarily ever being in that situation or even knowing anyone in that situation, and still have sympathy and compassion and understanding that as a society we are morally responsible to help those people. Republicans don't have that same decency and compassion and set of moral values that tells them that we as a society have to take care of those who are less fortunate.
The only way that someone like McCain can change is to be put in a situation that makes him one of the less fortunate, in this case, his brain tumor. While this brain tumor will probably kill him, he at least understands that with his great medical care he has a chance, but if he didn't have health insurance and wasn't able to get treatment, he would definitely die. So his own personal experience allowed him to realize that throwing millions of people off of health insurance and causing many of them to die was just evil.
The same doesn't hold true with tax cuts or fiscal policies because McCain doesn't know what it's like to be poor, or even in the middle class. Republicans just have no empathy or concern for others. As far as they're concerned, the poor are the ones to blame for their poverty. McCain's wealth couldn't prevent him from getting a brain tumor and that made him think about other sick people, but unfortunately we can't take away his money to make him think of what it's like to have to struggle to survive.