Brian, its probably cause VAG went cheap on the 2015 beam axle golf tdi's. I agree with you on other models.
I do not feel it is "cheap", after all, many FWD VAG products used twist beam type rear suspension, as well as many other cars. And I really do not think 99% of Golf buyers worldwide even know nor care what type of rear suspension their car has. I also do not feel there is some huge difference in the way the car drives under most normal day to day operations people need their cars for. The Golf is not a performance car, the GTI is a performance version of an economy/family car (which even those in the A1, A2, A3, and A4 trim made do with a twist beam rear axle... and arguably those are probably some of the most loved GTIs of all... I know I favor the early A2 8v GTI more than any of the rest).
Time will ultimately tell how the A7 Golf platform holds up. Unfortunately for us diesel enthusiasts on this side of the world, that means 2015 cars only. Which sucks, because we will never get to have enjoyed whatever minor changes the platform got during its tenure, although I do not think there is a whole lot.
I drove a 2017 Golf (with the 1.8L) last week for quite a bit, had to verify the water pump was bad (typical). It had IRS, and I do not think it rode or drove any different. I bet if you placed 100 people behind the wheel of that 1.8L gas + Aisin 6sp slushbox IRS A7 Golf and had them drive it back to back with a 2.0L TDI + 6sp DSG beam axle A7 Golf the ONLY difference ANYONE would have noted would be how much better the TDI RUNS and how much crisper the DSG shifts, and how much more passing power the TDI has on the highway. I suspect anyone who was given the choice of those two cars based on the driving experience alone would have wanted the TDI, with the exception of someone who is always cold and maybe drives short trips and would whine about the slow warm up time. I doubt anyone would say "man, this TDI just rides weird, and handles awful, and...."