Quite simply, Irish music had better PR.

Before the rediscovery of English music around 1980 most of the tunes played in sessions in England were Irish. Bands like The Chieftains, Planxty, Bothy Band, De Danaan and others were popular. Scottish music on the other hand had a fairly low profile and was mainly associated with the White Heather Club and Scottish Country Dancing, and seemed old-fashioned by comparison. The exception was Shetland music, which had higher profile, probably largely thanks to Aly Bain (and which some would argue isn't really Scottish).

Nowadays the scope of music is much wider. Irish music remains popular, and not just amongst those with Irish antecedents, English music has had a revival, and French, Scandi, Klezmer and other genres can also be found. Scottish music still hasn't captured the English imagination and seems still to be mainly played by exiled Scots.

I also wonder whether the Scottish enthusiasm for playing in A is factor in a session world dominated by D/G melodeons?