Why wasn’t the ACA single payer?
One word: obstructionism.
I don’t really have the energy to put a dog in the rest of this fight, but I did want to answer this question, in particular.
Obama’s original proposal was a single payer + private supplementation plan, similar to what’s in place in many European countries. The single payer part, in particular (among a myriad of other things) got gutted once the Democrats lost their filibuster-proof majority and the Republicans, who were hell-bent on obstructing Obama at every turn and all costs, were able to drag their feet and throw enough hissy fits to get the Democrats to cave on enough things in the name of getting something in place.