For me there's one simple reason - Synology NAS.
My storage and media at home (which is a huge collection) is on a Synology NAS and that's not about to change. Fact is that even though Synology has apps for Windows Phone/Mobile, they're crippled compared to their iOS and Android counterparts. You can use the apps so well to manage and interact with the NAS.
I've been waiting forever for some decent updates in functionality etc, but they never come. Updates come, but it's mostly just small bug fixes etc.
I've got an Android phone for work, and I hate it - it's like a toy, fragmented as hell - you never know where a setting is to be found, home screens are a mess - I just can't find myself in it.
I think iOS is boring and unspectacular, but it gets it done well, and clean - and it takes pretty good pictures in the meantime too.
So, it's not the app gap in and of itself (though there are some things missing I'd like), but rather the lack of support for a very specific hardware solution I'm invested in, which is likely to finally put the nail in the coffin of Windows Phone/Mobile for me.
Shame really....
PS: Synology, if you're reading this, PLEASE PLEASE provide a decent upgrade for your Windows Mobile users.