Umbra armor's effect is applied no matter why the enchanted creature would be destroyed: because it's been dealt lethal damage, or because it's being affected by an effect that says to "destroy" it (such as Nekrataal's triggered ability)
I also suspect that people really liked the motif of the sun growing closer to Bolas horns, which you can see play out in this cycle
All you need to do is banish that card from your deck face-up, then two turns later that card is all yours
three mana to draw five is fantastic, especially in coordination with cards that want you to fill your graveyard
This makes cards like Gray Merchant of Asphodel and Thermo-Alchemist especially deadly in 2HG, since their damage output is twice as strong as it would be in a 1v1 match
Few collectors manage to buy and sell the rarest cards of all time, but plenty are out there grading and flipping hard-to-find or highly desirable cards from more recent sets