Because Adobe did not develop it for the WP7 platform before they abandoned all development of Flash for mobile devices.
If you want access to media on particular websites, you need to ask those websites when they will be adding Windows Phone to their list of supported devices. (iPhone/iPad don't support Flash either, so the websites must have done something specific to
support those platforms.)
Nokia Lumia 920, EE UK, & HTC 8X, AT&T: 8.0.10211.204 Samsung Omnia 7, T-Mobile UK: 7.10.7740.16