[AlbertCory]

What @riskable said.

The list of "patentable subject matter" ("101" to insiders) is a legislative decision, i.e. a political one. The Congress doesn't need an ironclad reason, but there are plenty. The Constitution just says:

Congress shall have power… to promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries.

It's simply a matter of Congress using its power to decide that the "useful art" of software does not need patent protection.

Pure mathematical formulas are already excluded from patentability.

And now yet another election is happening where Congressional and Senatorial candidates are not asked to support or oppose this proposal.

This is how it's going to happen, if it ever does. A bill gets introduced, the Establishment runs op-eds against it, and the battle is joined.