The advent of specific touring bikes probably came from the specialization of bikes that began back in the 80's and 90's. As "road" bikes became more and more specialized and less and less suitable for touring use, designs that were purpose built for touring were created.
In the past even sports frames had sufficient frame and fork clearance for reasonably wide tires, long reach brakes, long enough chainstays to provide adequate heel clearance, dropout eyelets were standard and somewhat "relaxed" geometry the norm. You could mount larger tires, a rear rack, panniers and go tour on it. As sports framed evolved, clearances got tighter, short reach brakes became standard, eyelets disappeared and geometry got more aggressive. Hence the touring frame as a separate type.
I have a 1983 Trek 400, sold as a sports bike, and a 1996 Litespeed, also in the same category. The differences are just what I mentioned above. The Trek has plenty of clearance for 32 mm tires, dropout eyelets, long reach calipers, a long wheel base and rather relaxed handling. The Litespeed accepts 23 mm tires and not much more, no eyelets and more aggressive handling.
BTW, a currently available throwback to my Trek 400 is the Surly Pacer. Again, clearance for large tires, long reach brakes, eyelets, etc. It's not a pure touring bike but it certainly would work well for all but the heaviest touring.
My old 1991 Raleigh Technium with long cage derailleur was used for club rides and touring. I loved that bike but it was set aside for others. It still rests in my garage and is ridden once in a while, but I now prefer my LHT for long distance slower excursions and touring. My son and I built a Pacer earlier this year and it is wonderful. It has a triple chainrig and Nitto noodle bars. I use it for my daily rides with my buds and century rides. I love both of my Surlys. They are perfect --- for me.