The 12 is very predominant in the UK, and I think that the ammunition being cheaper, more widely available and available from 3/4 oz upwards has been a factor. In general, we see very few 'other' gauges. Some 20s, but very few 16s or 28s - and few 0.410s.
I think also there has (in the past) been a feeling that a larger bore shoots (i.e. 12) a better pattern for a given load. We tend to use 1 1/8 oz (32g) or less in a 12 except for wildfowl (which now require non lead loads) and for clays 28g (1 oz) is the most common with some (including me) using 21g (3/4 oz)
Above 12 (10, 8, or even 4) are also seldom seen here.
