The delicate mechanical AS of the R3 and R4 probably couldn't be implemented without hurting the camera's ability to withstand drops and impact.
From the press release:
"A shock resistant structure and stronger camera cone create a robust body able to withstand falls from 1m onto 26 surfaces, in compliance with US Department of Defense “MIL Standard 810F” (previous models withstood 90cm). It can cope with rough handling outdoors."
I don't see how AS/IS or the likes would withstand falls from 1m. Pseudo-AS ("AntiBlur") makes sense, though most of us know enough to simply crank up the ISO ourselves.