Originally Posted by Anon La Ply
Some of it was pop in its day for the upper classes and some wasn't - big difference between Mozart and Varese. Since then new genres have sprung up and classical music had to be recontextualised.
Another thought ... popular with whom? Classical came from church music and was popular with the upper classes. The peasants at the time would have played folk / indigenous music but you don't hear so much about folk music of the classical period because peasants don't feature so much with historians.
Half of the so called folk music of that period can be traced back to operas and other works of a century or two before. melodically anyway. i know people who have insisted that Oh, Susannah and Dixie are American folk songs when it's pretty clear that they were written for minstrel shows and are thus pop/theatre songs. These tunes get popular and then the origins get forgotten. And as to church music. That started getting infiltrated by the pop music of the day as early as the mediaeval period. cross pollination, the motet form serving as both sacred and secular. Most of the composers wrote both secular(pop) and sacred(church) music Palestrina included.