I spoke with a few mid sized app developers this last while, and with one exception all of them mentioned this as the main reason why they don't support WP.
It's because their main battles for market share are fought on iOS and Android, and anything requiring them to divert resources from those two fights is considered a sideshow that could end up harming more than helping them.
That's also why the recent talk of a more similar UI being able to achieve more engagement by developers is so misguided. Up until the point where you can run the same app on WP and Android, without increasing development costs at all, a more similar UI will do absolutely nothing to to spur app development on WP.