In the case of the former, its because the best choices vary not only by color identity, but often by specific deck building limitations: Seismic Assault and many other red effects want you to play an extra-high land count, while a black deck with reanimation and Tortured Existence obviously cares about discarding creatures
Moldering Karok Wins Out This deck, on the other hand, is a BO1 draft deck I 7-0d with on MTG Arena Premier Draft
Yet its not gone for good: on a later turn, you can cast it again from exile, this time for its full cost
If the drawn card has a Trigger effect, the card may be revealed to activate the Trigger effect
Blue Consecrated Sphinx This would be stopped by the fact that it cost except for the fact that every time I see anyone playing it, they're cheating it out
The chosen recipient must be able to legally be enchanted by the Aura, so a player or permanent with protection from one of the Aura's qualities can't be chosen this way