Neither was Gambit, but I bet you he's one of the first characters people think of when they think "The X-Men" thanks to the old cartoon.
Robin is far and away the most popular Titan, but the others (Beast Boy, Raven, Starfire, and yes, Cyborg) are just as linked to that brand as he is. They are THE Teen Titans to at least two generations of fans, and the Teen Titans are one of DC's more popular brands. Cyborg simply has eclipsed Static at this point, as much as some would rather that not be the case.
Thanks to multiple cartoons, and myriad of games he's appeared in as well. And Gambit isn't the xmen, he's just one of the ones that are remembered amongst the others. How often do you think cyborg would be brought up if people were to ask the first thing you think of when you think teen Titans? Statics name is at this point synonymous with his TV show to the point that most people think his name is static shock.