Of course it was a tax, it was always a tax. They just tried to sell it as not a tax.
That's the point.
The Dems lied in order to pass the ACA bill. They were asked about the mandate, and they responded with, "It's not a tax. if's a fee, or a fine for not having insurance. We charge people fees all the time to pay for government programs."
Then, when challenged on the Constitutionality of the mandate (Congress can't pass a law to collect fees -- only taxes), they changed the story to, "Of course it's a tax."
This is the unconstitutional part. They violated the Constitution, then changed the LAW, and then acted like it was all good. In reality, the entire bill should have been struck down by the court. Congress should have been told to redo the ACA with the proper tax language in it. Nobody voted on a tax bill. They voted on a bill without any new taxes.
They didn't want to start over, because it was difficult enough to push it down our throats while they had control of the House (100% of Dems voted yes, 100% of Republicans voted no).
This would not be such a problem if the ACA didn't represent the largest tax and spend program in US history, and a jumping off point to push the country toward single-payer healthcare for all. Single-payer (or government-run) healthcare couldn't succeed until the insurance industry was destroyed. Obama said exactly that in a speech he gave before he ran for President.
Also, there has never been such a massive tax plan passed by a single party vote. They always had bipartisan support for the bills, because it's understood that something so large and complicated needs buy-in from all sides to be successfully implemented. We saw how screwed up the Obamacare roll-out was. Had to delay it for another 6 months to fix the basic websites.