Even when Dems have control of a state, they don’t because of infighting and sometimes more nefarious reasons. See the so-called independent democrats of New York State that have colluded with Republicans to deliver a very blue state to conservative factions.
Rendon himself has taken $100,000 from pharmaceutical companies and $50,000 from major health insurers. Single-payer would clearly hurt these industries, putting some entirely out of business.
Progress is very hard in a political system ruled by money.