I guess I should expand why not a laptop, but it boils down to removing lots of little friction points that make ephemeral ideas harder to put down.
1. Touch first is important because your instruments are touch as well. Being able to record my live instruments and not have to switch to a trackpad to move between UI elements seems small, but it really keeps me in the zone.
2. Same point above, but basically being able to switch between my live instrument and an on screen virtual one is a big deal to layer up an idea. Can’t do that easily with a trackpad or mouse.
3. There’s something to be said for the mobile platforms single app as a focus model. I know you can full screen a Mac app, or split screen on an iPad, but having a singular focus helps reduce distractions.
4. Weight/portability are quite different. Maybe a MacBook Air would be better, but I take my iPad Pro to more places with me than I take my much heavier MacBook Pro . I’m already taking it to draw on the go, so this is just one more thing I can do without needing to buy a new device or take a heavier one.
Edit: oh and number 5 is a big one. I can put my iPad Pro in my existing music stand that works for me at multiple heigh levels. There are laptop stands that might work too, but I already use my iPad for sheet music, so it’s already a convenient setup.