We do have a tag for this game, ufo-defense, but I'm guessing that the user didn't find it because he was looking for the name the game is commonly referred to as... simply X-COM.
Looking at the questions that use the ufo-defense tag, three of the five unnecessarily have the x-com-series tag as well. If you count the one referenced above, which I already fixed, that would have made 4 of the 5 questions about X-COM: UFO Defense that had the x-com-series tag. There are a few questions whose only tag is x-com-series, but they're clearly questions about a specific game, not the series as a whole. It implies to me that people start putting in a tag for their questions, their inclination is to start typing 'x-com" and they're not finding the ufo-defense tag.
Note that X-COM: Terror From the Deep uses the tag x-com-tftd. I didn't see any tags yet for X-COM: Apocalypse, X-COM: Enforcer, or X-COM: Interceptor, though I did find a question for at least one of those which was asked under the tag x-com-series.
There are two new X-COM games slated for release this year, and I think that "X-COM" is an important part of the tag. However, they didn't pick terribly distinct names.
- One, a first-person shooter, is simply named XCOM.
- The other, a remake/re-imagining of X-COM: UFO Defense (also known as XCOM: Enemy Unknown) is simply going to be called XCOM: Enemy Unknown. Hooray for name collision.