It wasn't the 90's. At least not until the end of the decade like 98, 99 when the internet was becoming more common place and napster and file sharing were beginning.
At that point, if you then wanted to show your friends a "viral" type song, it was a lot easier to spread the word. Before that, U Can't Touch This, Macarana, Livin La Vida Loca were just popular by being played and requested a lot by people. Frankly, sometimes it's just nice to revel in a silly song.
By the end of the 90's start of 2000's, you had stuff like The Hamster Dance, Blue from Eiffel 65 and Who Let the Dogs Out taking off in part, I think because of the internet making the music more accessible. This then led to other phenomon like Rebecca Black's Friday and Gangnam Style by PSY taking off as the internet became more prolific and sites like YouTube further made it easier to find and spread these types of songs. Of course, you could have someone like William Hung become a viral sensation just from people seeing him on TV audition so it's not like the internet is the only cause of it. Sometimes, you just like to laugh along with something and then the joke dies and you move on.