I, too, am unfamiliar with the exact history of the phrase but I have some decent ideas (especially from living in the Bible belt) what it’s supposed to mean.
If I understand correctly, this phrase may have been tossed around as early as the 70’s and certainly the 80’s. It is sort of an attack on very structured, liturgical sort of churches like the Catholic church, but also several “high” prot churches. The phrase “personal Lord and Saviour” is meant to wake people up to having a real relationship with God for themselves, as opposed (and I can only guess) to having an impersonal Lord and Saviour. I say this half-jokingly, but it has some truth to it. The phrase is meant to cause people not to depend on religious organizations to communicate with God for you; you do it yourself. For example, many accuse Catholics of relying on the priest to get them to heaven, or Prots relying on membership to their denom to get them to heaven and so on. It’s an attack on what seems to be impersonal, rigid, cold organized religion. Instead of all that, they say, have a personal relationship. YOU do the talking, YOU do the praying, YOU do the reading of the Bible and so forth.
There has also been preaching that people sit in church Sunday after Sunday for years and years, and never “accept Jesus in their hearts” hence they don’t have a personal relationship with Him and end up going to hell.
These things, I think, are at least partly behind that phrase.