They don't make it appealing enough...
They want developers; but they also want developers to cough up quite a few bucks before they can actually do some tinkering with their phones, and I think that's where MS is missing the point.
I own a WP7.5 device, I like to tinker and I also have a fair amount of experience with C#.NET and VB.NET. Needless to say but I picked up the previous SDK and was actually quite pleased with it. It gives you the well known Visual Studio look, gets you a graphical phone display where you can setup your visual components and it gets you the emulator.
But here's the thing; messing with my phone is a whole lot more fun than messing with some emulator. But I can't do that because my phone is "dev locked"; iow: you can't hook it up to your PC and try to gain access to it, won't work.
And to unlock it, you guessed it, I need to cough up some big bucks.
That really doesn't appeal to me. I want to learn the environment, check how stuff works using MY phone, and I really don't mind coughing up, say, E 10,- / E 20,- to cover administrative costs which is bound to be involved with getting me an unlock for my phone.
Instead my choice is: Either you jump in fully or you can forget about it.
Chicken and the Egg: before I can decide if I want to jump in fully I'd like to gain some hands on experience. But in order to gain some hands on experience I gotta jump in fully.
Guess what? I'll simply not jump in at all.
Microsoft needs to make it more appealing if they want to get the interest of developers... Sure, you'll always have plenty of fortune seekers; but they come and go. Something I'm sure MS is going to find out soon enough.