It could be for several reasons. Maybe we simply haven’t heard of it or tried it. Suggest an addition on the BJCP Forums in the appropriate category and we’ll look into it. More typically, it’s because we have a sufficient number of great examples. If we have several widely-available examples, we won’t add a beer that is only locally distributed or only brewed seasonally. We relax this rule if we don’t have good examples available.
We try to list examples that are distributed in the US, although we will list some beers that are only found in limited areas if those are truly the best examples. Düsseldorf altbiers are a great example — we list several that are only found in the Altstadt in Düsseldorf. Some beers are no longer found where they originated (e.g., Vienna lager). Other beers have no good, widely available, modern commercial examples (e.g., Classic American Pilsner).
The final reason for not listing your favorite beer is that perhaps we don’t consider it a classic example of the style. It may be a great beer, but not very representative of the style. It might be outside the style parameters, or just not be a great beer in our opinion. Keep in mind that just because a brewery decides to name their beer in a way that seems to indicate a style, it might not actually be brewed to that particular style. In Canada, there is a popular beer named Alexander Keith’s IPA that most definitely is not any form of IPA. Usually it doesn’t come to this type of selection process since we are finding and adding great examples all the time.
Although we do attempt to list classic examples from the country or region of origin, we will list good US versions of world beer styles. Likewise, we might have beers from other parts of the world that are good examples. There are several great German-style beers from Eisenbahn in Brazil, for instance.