It's probably the immediacy. You click an app and you get a fullscreen touch UI with no distractions. Quite different to opening a slow-loading DAW and starting up various plugin windows inside it.

iPad music apps are typically priced far lower than the equivalent PC apps, and there's a thriving community of iOS-only development as well.

For me it's the sweet spot between hardware (which is expensive and annoying to cable up) and PC VSTs (I associate my laptop with work). The fact the iPad can also be used for videos/books/drawing/note taking is just a bonus.