Because they couldnt do it the way a bill is supposed to be passed. They didnt have the support for it. They wouldnt allow Scott Brown to be seated because his vote would stop it, and secondly, the ACA wasnt a budget bill so it didnt qualifiy to be passed through using reconciliation, and lastly It was a spending bill, and all Spending bills must originate in the House, and they simply passed the law that was written in the Senate.