Manitas has said that it also can be the case with Irish music. I've heard of these reel only Irish sessions but have yet to go to one like that. My regular session in Norwich pretty much alternates between reels and jigs but even that largely forgets hornpipes, slides, polkas, etc.

I'll throw a thought in from the "Irish" session view and risk getting flamed... There are times that it seems to me that reels are regarded as the difficult tunes (a contrast to my thoughts regarding jigs and 5 string banjo perhaps being difficult) and the "better players" like to be seen to be playing the better or more demanding tunes. On the other hand, there are many polkas that are really easy to pick up and play. Perhaps their accessibility can get them cut out as perhaps is the case with hornpipes such as Harvest Home....

Jon