This is because of the locke bootloader. You cannot flash custom ROMs. The bootloader will check the signature of the ROM, and won't flash it if it's a custom ROM.
Windows Phone proves that you can make an original and innovative OS without copying Apple. Something Google didn't manage to do.
NOKIA Lumia 920 | OS: WP8 Apollo (8.0.9903.10)
NOKIA Lumia 800 | OS: WP7.5 Mango (7.10.8773.98)
HTC 7 Mozart
HTC HD Mini (sold)