Out of curiosity, why would one add a new type as opposed to putting a switch in the existing type? Same question would apply to some of the amp types - I'm guessing there is a deep engineering reason, but a few sentence layman explanation would satisfy my curiosity just fine.
I think it's because a knob or switch with a variable effect results in a more complex model than a fixed circuit. In the models you can take several different physical components and simplify them to one resulting affect on the audio that passes through it. When you have a switch now you have multiple different parts loaded into memory you have to swap between, and you might need to use more steps in your model to simulate the parts around it. So the Axe FX tends to use an approach where it assumes that switch is just a fixed value in the circuit. Then a different model is created with the alternative fixed value in the circuit.
This also shows up in some amps where sometimes a switch just adds a capacitor on an EQ circuit to shift it, and sometimes it adds and entire circuit with additional gain stages.
Some amps have basically three different signal paths, one for clean, one for crunch, and one for lead. Some just sub in extra parts in the middle for lead. The Mark IIC+ is an example of this: Clean it's similar to a Fender clean circuit, and when Lead is engaged it adds a couple more gain stages into the middle of the path. But the same tonestack is used, and the parts before and after the lead circuit are identical too. It could be considered just a switch that adds more gain instead of a whole separate channel.
But that's a lot of variety that the Axe handles just by saying "we'll pretend there's an amp without the modes or channels and instead is just handwired to this config, and then we'll pretend there's another amp which is handwired to this other config." It's a clever solution because it keeps the models simple and easy to create (compared to other approaches). And the things that are implemented as switches instead become available on all the models.
So I'm assuming why it wasn't added as a switch was because the Low Peak/High Peak switch is kind of specific to this pedal, where most pedals just choose one value for the thing it toggles. And it didn't make sense to make it generic for all the models. But the bass shaping control on the Precision Drive
did make sense to be generalized to all the models, so Cliff was good enough to add it to the drive block so it can be applied to every drive.
