Because "for all" is generally taken to mean provided by the government, and history has shown that without functioning markets government guarantees are ultimately worth the paper they're printed on. And if we actually had a functioning market, then why does everybody need to go through the government, as opposed to only the people who actually need the financial assistance?

We don't take "food for all" to mean that supermarkets are paid by the government. Rather there is a functioning market for most people, and subsidies to help those who cannot afford it. The competitive market keeps the overhead from getting out of hand and makes it so most people have the autonomy of paying for themselves on the spot, while the government provides a safety net for those who cannot.

Now having said that, the entire healthcare system is presently a dumpster fire. The other side of the aisle has done very little to address this dumpster fire with market based solutions - eg make providers publish full price schedules that are uniform for all payers, issue estimates for all non-emergency care, prohibit providers billing arbitrary amounts post-facto, and strongly punish any fraudulent billing. Such reforms could certainly exist alongside increased access to health "insurance", but that political team has been mostly spreading FUD to keep the status quo rather than proposing solutions. Meanwhile Medicare is actually pretty good, it seems that Medicare For All is the only train leaving our current situation, and it therefore makes sense to get on it.