You really shouldn’t be surprised! It’s not that there were no warning signs along the way. Microsoft made it very clear that you will not be able to use the Windows Phone Marketplace once the changes are in place and your phone is still running a version of Windows Phone below 7.5.
So, as of recently, if you can’t download, buy, update, or review apps in Marketplace — both on the phone and on the web– it’s because you didn’t update to Windows Phone 7.5. Of course, you might be running an earlier build of the platform and still be able to access the Marketplace; this will soon change as the modifications are rolling-out over a couple of days. Windows Phone 7.5 (aka Mango) was released last fall and all OEMs and carriers should have an update available so make sure to check for its existence and apply it if you haven’t already!