Are Tass and Pravda allowed?
The obvious reason is that Fox and C-SPAN reporters ask hard questions that make their political hack candidates look even more like the out-of-the-mainstream crackpots they really are. No great loss since these preliminaries are just beauty contests until they start counting delegates after the primaries. I expect the DNC will rig the whole thing for Biden like they did for Clinton so they are all just wasting their time.
MSNBC is Tass & Pravda