Audio apps tend to need to have special UIs and widgets which aren't typical needs for most apps. You need special sliders, drawing tools, and color schemes for live performance that aren't baked into many widget toolkits. Even if they are, you're dealing on such microscopic levels of detail on these UIs that you really do want them to be the same pixel-for-pixel on any machine. And if you're selling a cross-platform audio plugin to musicians for a couple hundred dollars for the last 5 years, it's nice to totally remove OS-specific UI weirdness from the equation.

Here's an example UI showing my point (and the app Juce was spun out of) http://www.tracktion.com/