Well they haven't really. Moving a "Metro" Silverlight project to WinRT requires very little effort at all and can easily be absorbed into an upgrade release schedule - it's not like you have to do a major re-write or change the language or anything. And from an end-user perspective, it doesn't matter as long as everything still runs as it did before. Some apps won't make the best use of my phone, but heck that's true of some of the original WP7 apps that haven't been updated to enable Mango specific features too.
In the grand scheme of things though, not caring so much about a breaking change here and now, when the userbase is relatively small, is a far more sensible decision than being lumbered carrying the burden of supporting software that you had the option to drop at an early stage with minimal impact (whether that be Silverlight, WinCE or whatever).