But of course the real (and perhaps obvious) difference is that Apple is much more strict about its ecosystem, having decided ARM will definitely be a thing and developers need to follow to stay relevant.
The Windows version is "maybe there will be ARM devices one day, maybe". That's not nearly as convincing.
I believe the most convincing thing for developers (besides rewards like promotion and filters in the store, which MS badly sucks at) would be a wave of serious ARM devices coming out in the very near future. The longer it takes the slimmer the chance that Windows on ARM will really catch on.