I always assumed it was due to gluttony being typically visible. Reading 1 Corinthians 6:9 and following, for example, you see several sins many sins which are more internal, or at least easier to hide. Sitting in church and looking around at the crowd, you can get an idea on any gluttonous individuals just by their look. Not always, and clearly you can be overweight without being a glutton. This though was why I reasoned we never hear sermons on this particular sin.