Why? Because Microsoft admitted it was doomed by design speaking in an interview with CNET UK.
...according to Greg Sullivan, senior product manager for Windows Phone, Microsoft knew all along it would be orphaning Windows Phone 7. Asked when it started developing Windows Phone 8, he reveals development work was going on in parallel with Windows Phone 7 -- and even earlier.
read the full article here.
If you are planning to develop for Windows Phone 7. Don't. Instead, go with the yet to be released Windows Phone 8.