They have. Here's why.
The Affordable Care Act has literally gone through all three branches of the US Government. It went through the House and passed. Then it went to the Senate and passed. Then it went to the President, who signed it. Then it went to the Supreme Court, and it was upheld. It has been quite literally been done by the book, and it's situations like these that were foreseen by the people who wrote the Constitution, which is why they wrote in three branches of government.
But the GOP in the House, for some reason, think they know better than the legislative, executive, and judicial branches, as well as the founding fathers and the Constitution, and have decided to take matters into their own hands and nullify legislation they think shouldn't be law.
What in the world.