Reason? Simple, really. These apps use undocumented APIs to access the Pandora service. Pandora, for whatever reason, changes them on occasion breaking app functionality. Are they doing it on purpose to disrupt these apps? Who knows, maybe or maybe not. What we do know is that it means you'll have to wait till both developers update their apps to fix it and from what we've heard, both devs are hard at work on fixing it.
Until then, MetroRadio is "hidden" from the Marketplace and RadioControlled has submitted their fix (and "future proofed" it against this type of situation occurs again). Stay tuned though, we'll give you a tour of the new working Radio Controlled soon enough.