I think the term ceilidh was adopted as, in the 1970's, some other entertainment was provided eg a singer or a morris display spot. It also served, in England, to denote that the dancing would not be of the Playford/Country dance style but would be a bit rumbustuous with more time for addmg stepping in. There is also the problem that, again in England, the public has come to identify the term barn-dance with American squares and contras.