Because it is already law.
If it's a tax, no, it's not. Tax bills have to originate in the house. Obomacare is a senate bill. It may have "passed", but it wasn't created following Constitutional procedures. Therefore, it's unconstitutional as a tax if the procedure itself is argued.
If the Republicans wanted to stop it, they could. They could simply defund it. It's not valid unless it's re-written by the house and brought up by the house for a re-vote.
It's just that Republicans like it (I.E., it's the good 'ol good cop/bad cop routine).
Remember. Washington sees us as a slave force and not as human beings. We're dollar signs. In Washington, it's all about power and money. It has nothing to do with us. We're like the public school kids - slabs of meat on an assembly line. Each one has a dollar value. We're born, we pay, we die.
I believe once a politician spends too much time living in the beltway, they lose all their sense of humanity. When they need our attention for some reason, they become like actors on a stage. They say they're doing everything for us. Yes, it's all for us (and the ignorant audience cheers them on).