I think the problem may be that this film was just released a couple years too late. In 1991, Bill and Ted already seemed "so '80s". Even though the '80s were only a couple years ago back at that time, the landscape of the music and style for kids had changed so radically with gangsta rap, hip hop, Pearl Jam, Nirvana, grunge and the Seattle sound. Bill and Ted with their Ozzy Osbourne, Van Halen and Guns N' Roses music along with their '80s style seemed so out of place and very outdated in '91, and I think that's one BIG reason the film bombed at the box office. Nobody but surfers were still saying stuff like "excellent!" and "bogus!" in 1991. "Gremlins 2" which also came out in the early '90s suffered a similar fate of being a good film that bombed at the box office because it was too associated with the '80s. The transition from the '80s to the '90s was a much faster change then now with the '90s and '00s. 1991 was nothing like 1988 or 1989, whereas right now, 2002 and last year 2001 still looks/looked like 1995 or 1996.
If only "Excellent Adventure" which was made in 1988, was released THAT YEAR instead of 1989, and "Bogus Journey" was made quickly and released in 1989, then it too would have probably been just as wildly received as the first.