Looking at it from a developer's perspective, this is actually valid! Pretty much everything Apple does takes off, and their customers open their wallets very easily.
Compare that to W10 and W10M: so far consumers have mostly ignored apps on the desktop, and WP/W10M is a fledgling OS that has failed to take hold after 5 years in the market. Its future as a major player is anything but secure. I do not think it is going away anytime soon, but neither will it be a significant force for the next couple years at least.
Looking at it as a developer who needs to put food on the table, which one would you choose? If you do not need the income, W10 makes sense. With a smaller store, you have a better chance of getting noticed. But, developing for Apple is much more secure, relatively speaking.