Why some useless cards are very rare?
Because some cards are discontinued cards and Konami don’t have a plan to release it again or in a near future. For Example : Cards from USB Duel Pass are availble only at USA this is why Dark Paladin is very rare, Red Eyes Black Dragon Green Background only release with Shonen Jump May Edition (during 2005/4/28) and YU-GI-OH Duel Monster Beginners Pack 2005. Some cards is very hard to get such as Thunder Kids, Haniwa, White Dolphin It’s random Rare Loser Cards (Cards you will get when you lose in Events) but very hard to get it.
