> what makes software unique in that it would be effectively excluded

I can answer this:

1) Because software is already covered by copyright.

2) Because any given idea or concept in software can be implemented an infinite number of novel ways. Patents only cover specific implementations of an idea... Not the idea itself.

Let's use Amazon's One Click patent as an example: How many different ways do you think that could be coded/handled? How many different programming languages could be used to make it work?

If the One Click patent was actually specific enough to not be a broad concept (as required by patent law) it would include the actual code that makes it work. That's the software equivalent to an engineer's blueprint. However, if you look at the patent claims (https://patents.google.com/patent/US5960411A/en) you can see that every single claim is just generic nonsense (e.g. "The method of claim 11 wherein the client system and server system communicate via the Internet.").

Even if you add all the claims together and examine the patent as a whole you still get nothing but a broad concept on the idea of clicking once to place an order. Repeat after me: PATENTS ONLY COVER SPECIFIC IMPLEMENTATIONS OF IDEAS, not the ideas themselves. Which is exactly what every single software patent that exists defines: Nothing more than a broad concept.

You could require that patents provide the actual code that makes them work but then they'd be worthless because any given bit of code can be implemented an infinite number of ways. It would be trivial to change a for loop to a while loop or wrap things in functions or even something as simple as using a different programming language. Any of those things and more would get around a patent on a specific implementation of software, aka "code".

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.

We’re still in the first decades of the profession. You should get a patent for describing a solution that would have gone unknown for twenty-ish years, not a land grab on the easy problems we haven’t looked at yet (or weren’t worth writing down).