It was a box office disappointment. Clue seemed to confuse audiences and critics alike, both of whom doubted that a film based on a board game and with the gimmick of different endings could possibly be good. Despite being packed with a talented cast, the film fell by the wayside in the mid 1980's.
Rather than carry on the conceit of different endings for home video, Paramount combined the three endings (which required purchasing three different tickets to watch in theaters) by suggesting that the first two were possibly how things happened, but that the third one was the truth. Unleashed on the video market, the film began to take off, in particular with the attendees of "Camp Latchkey" who would wear out VHS copies of the film and catch it every time it aired on basic cable. Cable channels found the film to be a nice mix of adult seeming situations that were mostly family friendly, so it was placed into heavy rotation. The film built up a huge audience.
So what made the film a favorite at Camp Latchkey? It is undeniably repeatable and extremely quotable. Plus, what kid wouldn't want to be in a huge mansion, solving a mystery? Clue might not have set the world on fire at first, but it has certainly made up for lost time over the last thirty years, building a huge cult of fans who have embraced the screwball film.