9. It's The Only Royal Rumble With No Form Of Championship Matches
The 1988 Rumble saw The Glamour Girls lose the Women's Tag Team championships to The Jumping Bomb Angels. A year later, Haku and Harley Race fought over the "unofficial" kingship (in the form of a crown and accessories) of WWE. Every Rumble from 1991 onward featured at least one World title match, in some form. That makes 1990 the only Royal Rumble that had no matches fought over a physical championship or token.
The Rumble'sundercard featured three singles matches and one tag team bout, and only one of them (Ronnie Garvin and Greg Valentine's hard-hitting submission match) had any substantial backstory. The event was sold on the allure of the Rumble match itself, and the many colorful stars that would inhabit its hour-long duration.