Well, I’m glad you asked. 🙂
The reason is that for a few hundred – no, make that a thousand or so – years a lot of believes believed that there was one, universal visible “church.” Like a lot of things, this visible church started out united, but became fragmented over the years leading to the development of the Roman Catholic Church, Eastern Orthodox Church, the Coptic Church, the Nestorian Church and, eventually, the Protestant Church (or, should I say, churches).
Unfortunately, this fragmentation did little to change the view that there was one ‘visible church’. Instead, folks simply assumed that THEIR church was the ONE, and everyone else was not. (sigh)
Sometime during the 1500’s this view (thankfully) began to change – as noted in the Westminster Confession of 1646 which states:
The catholic or universal Church, which is invisible, consists of the whole number of the elect, that have been, are, or shall be gathered into one, under Christ the Head thereof; and is the spouse, the body, the fulness of Him that fills all in all. (Ch. XXV)
However, this was not to be the end of the debate (double sigh).