Dynas put the rubber mounted big twin like the touring bikes used into a lighter simpler better handling chassis which also made it the least expensive big twin.
The old Softail's chassis didn't work with the rubber mounted engine so it had to be rigid mounted which meant it had to be counterbalanced. This drove the price of Softails up even more since their engine was unique to the line. Add in the function follows form compromises of the old Softail chassis it's weight and it's very limited lean angles and it was a whole different machine from the Dyna.
But with the new M8 engine the opportunity was there to combine the two by designing an entirely new Softail type chassis with all new geometry, less weight, far better handling and lean angles than the old Softails. So the Dyna would no longer have those advantages and combining models with names and styling form both previous platforms streamlines the product line.
Bottom line is they made a Softail that handles like a Dyna or a Dyna that looks like a Softail works both ways. And with Harley Davidson adding new lines in recent years like the Street 750 and Trikes trimming out the V-Rod and Dyna keeps overall cost reasonable.
