I was super confused as a kid, because I had no idea who any of these characters were (aside from Spongebob, Plankton and Jimmy); never saw any of these series before and though they were made for this game.

Could be wrong on this, but I'm pretty sure Nickelodeon never aired any of these series in the UK, and if they did it clearly wasn't enough. Also all of these series (except SpongeBob) we cancelled when this released, so that might have had something to do with it (can't rerun old Invader Zim episodes, that's precious time that could be given to SpongeBob, it's not like that already took up 70% of the networks broadcast time...)