It's just that when you get past the touchy-feely "Nobody should be poor! Everybody should have health care! Why can't we all just get along‽", and down into the real world of economics and finite resources, it's not so easy. It isn't all that hard to create a universal health care system that is worse than the one we have now, and there are all kinds of other issues, too.
You might get me to sign up for health-care vouchers provided from the government if you really insist, but all current proposals seem to just beg for further increasing costs as the health care industry is further isolated from pricing signals and beg even harder for regulatory capture within five years by the already-powerful health care lobby. (The idea that the medical lobby will be defanged by centralizing all the power is so absurd I can't believe anybody who can think the words "regulatory capture" can believe it for more than five seconds without bursting out in laughter.)