It might be something to do with the number of players and the place as well though. I mean, If there´s a small bunch of well balanced instruments and everyone can hear each other you´ve got room to respond to each other or just accompany whoever is leading the tune. You can play around with dynamics too, even vary the tempo. As a result good musicians can be expressive. When only two people next to you can hear you properly theres no room for that so everyone just has to blare the tune out and play fast so theres some element of challenge to the whole thing.
This isnt just a huge denigration of sessions. They can be great fun and occasionally really magical musically, but if they´re huge and theres a massive noisy crowd, sometimes youve just got to give up on the expressiveness and play loud fast music, (which is probably what the crowd wants anyway.)
Also, when I busk I notice that the faster I play, the more money I get. (Within limits). The man on the street as it were just wants fast happy sounding music.
There are some tunes that just have to be fast though. I mean just imagin Martin Hayes playing the Mason´s Apron. He´s a brilliant player but that would be really bizzare.