Diana started making match rifles with the totally recoil-free double-piston system in 1963, with the model 60 (two examples of this in pic below), then improved the basic break-barrel design via the models 65 and 66. These had been around for 15 years before the model 75 came along.
But you are quite right, in spite of the excellent powerplant, Diana seems to have conciously decided to go for the lower end of the market when they decided on a break-barrel format. The fixed-barrel Anschutz 220 came along in 1959 and the FWB 150 came out just after the model 60.
By the time the model 75 was introduced, I believe the FWB's were just too well-established, and Diana's reputation for making entry-level match guns just too deeply ingrained. For whatever reason, the 75 was never an Olympic-level favorite in spite of its good qualities.
It's important to remember that the market for target airguns in Europe is huge, though, at all levels of competition and price. The 75 didn't win many big international prizes, but Diana sold a ton of these guns. The 75 actually stayed in production longer than any other spring rifle except the FWB 300S.
