But can someone explain why the EM is so revered?

Fireball was so successful they made Fireball Classic, Fireball (Home Version) and Fireball II. Players and operators really liked this machine. There are a few Bally machines from this period that are highly collectible - Fireball, 4 Million BC, Captain Fantastic, Wizard, Old Chicago, and Nip It. All were very successful for operators, and this was because they were GREAT machines to play.