They make the decks make sense both mechanically and thematically
Its no surprise that Tekuthal, Inquiry Dominus is heavily played in EDH given that it combines these two mechanics
Having someone start with Leyline of Resonance on the battlefield probably means a shorter game
These conditions activate both at the end of your turn(s) , but, in the case of poison, it also takes effect at the end of your opponent's turn(s)
Others attempt to replicate genuine rare cards to deceive serious collectors
That aside, there's an extra point in the rules that states sponsors have the right to disqualify entries "not consistent with the spirit or theme of the Contest." Regardless, it's not been officially confirmed that this is the case or if it's the reason certain entrants have been removed from the Contest, so it might be a different issue entirely