Because frankly, the senate rarely screws up this badly.
Generally, the Senate takes a house bill they don’t like which included some revenue items, and their first step is to “amend” it with a replacement. Then, whatever they do, they are simply “amending” a house bill, so origination is not an issue.
But here, they started with the bill in the Senate. So once they added revenue to it, they violated the constitution.
The beauty of this approach is twofold. First, it is a simple majority vote, without much time to mess around. Second, by declaring the senate bill invalid, there would be no bill to “reconcile” later. The senate would have to pass some other bill first. Which would likely mean the senate taking the house bill and amending it.
They may “Deem” it passed like Obamacare. They have tried that trick before.