Note: This is my own opinion
Poison type needed a legendary of its type. If we think in a different way, poison effects are also evil. Dark already has legendary Pokemon of it's type but not poison. Poison is the last to receive a legendary Pokemon of it's type (if you count Genesect as a legendary or else 2nd last).
If you come to type match-ups, both crowned Pokémon are immune to 1 or both of it's STABs (but Zamazenta still resists dragon type).
Also dragon types are supposedly fearsome creatures who cause a lot of damage so I think that makes sense too.