So why not flip the switch then?

hardly any SE sites have a community worth paying for, which is kind of the whole point

Well there's plenty of sites owners (with credit cards) that might assume otherwise. It depends on the target audience, but if it was for say doctors or lawyers 1/10th the traffic or even 1/100th the traffic of a typical programming site would be a success.

Maybe they did a survey among all the sites to come to your conclusion, but it seems odd they never tried go paid.

The people that could afford a site aren't the people that answer questions for free.

The main problem was that there was no way of charging the people who asked questions - who were the only ones likely to want to pay.