The EARTH attribute monsters are currently the most common attribute in the game. There are plenty of lower level monsters that are EARTH, but not many high levels yet, and even less than that are viable to use in most decks. While there are a lot of monsters sharing the same attribute, there aren't that many EARTH support cards to be used, since most of them are already quite sturdy from either good base stats or useful effects.
List of EARTH attribute monsters
For details of each card, tap the name of the card.