Doesn't the issue at bottom boil down to this: A single payer system requires that you go toe to toe with Big Pharma and Big Insurance, whereas targeting the substandard employers doesn't bring the drug companies and insurers into the fray? In fact, they stand to benefit from the extension of employer-paid health coverage, as would many employers who feel they are at a competitive disadvantage with Wal-Mart and the other firms who are presently undercutting them on health costs.
If this is so, then as with all struggles, the issue turns on an assessment of the relationship of forces. Clearly, those favouring single payer think there is enough popular support which can be tapped - given sufficient will by the unions and their allies -to mount a powerful mass campaign, while others think any such effort would again be crushed by the industries with a direct stake in the outcome.
I'm sure this is how the US labour movement views the issue. Whether it's assessment of the possibilities is appropriately prudent or too timid is another question.