Marketing?
Nope, lots of people who had never heard of UFO:EU before and played it for the first time in the 2000s still liked it. There were no adds/commercials for the game around when I first played it (~2004), either. Unless you count people's opinions about the game, and them posting those opinions here on this forum, as marketing. UFO: EU is a genuinely good game and that's why people like it.