Simple: Because there are no WP7 devices using the same processor/radio hardware. In addition to this, WP7 is not open-source, which makes the job of porting to this processor and radio stack almost impossible. In my opinion, it will never happen. The HTC HD2 (Leo) is currently the only device that has been able to run unofficial WP7 (as said by Cotulla, anyway). This has only been achieved due to the HD2's EXTREME similarity to WP7 handsets that are already on the market (it's hardware is very nearly identical, in fact).
But this...THIS is almost like it. ALMOST. Downside is that its a theme, so if your not used to it, and/or there are certain bugs, you can't turn it off so simply. Installation isn't as simple too. Oh and it supposedly drains battery faster as well. http://forum.xda-developers.com/showthread.php?t=859799