The full press release describes the reason for the change in greater detail. It wasn't just about the weight of the rig, but also the speed and deformation of the test rig. See picture below:Originally Posted by IIHS
They found that their test rig was good but not great at simulating the true interaction of test vehicles with their test rig versus what was observed with real-world heavy vehicles hurtled at a test vehicle. From reading the press release, this new test rig is one of many they are testing in order to ensure their new test rig really does reflect real-world conditions accurately without the extraordinary expense of destroying a Honda Pilot every time they want to do a side impact test.
Crash testing really has come a long way over the years, it makes me happy to see them validating their own test data against real vehicles and real-world crashes and admit things like a 3300 pound rig simply being totally insufficient in this modern world of SUVs. We'll all be safer for it.