Not all devs see a future in the WP market so MS has a huge task to actually get more devs on board and giving them a sense of security when it comes to monetizing..
Another point is that MS has very strict rules as to which apps it will allow in the store and in what form..
Some devs just can't deal with the extra security measures MS has in place...
Personally I don't mind those measures, because WP is one of the safest phones ever

I do miss some mainstream apps, but they will be worth the wait I guess..
Telegram is supposed to release an official app..
Whatsapp was supposed to give us huge updates...
So right now most of us are guessing that these devs are waiting for the official release of 8.1