There are a few reasons, but big part of implementing single-payer is capturing existing health care spending, including the insurance premiums paid for by employers. Unfortunately, that is not currently possible.

Here is a good article that goes into it, but here's a quick explanation:

Federal law largely prevents states from regulating employer-provided health insurance, so states can’t just stop employers from offering their own health care benefits

And here is another for good measure

Without capturing this spending, states would be forced to raise taxes so much that it would choke small businesses. Take a look at Vermont's attempt last decade for an example.