Because the nature of the current system has incentives for employers to have a healthy "pool". When you have larger pools with a single payer, that goes away.

Because the nature of the current system has incentives for employers to have a healthy "pool". When you have larger pools with a single payer, that goes away. With the current system, those people will end up either getting care that isn't paid for and gets picked up by taxpayers, probably not enough to survive. Or, those people will be shunted to high risk pools, which will be largely paid by taxpayers.