Or maybe it's because the modern bikes have better suspension, better brakes, and better acceleration, all of which is more likely to put more pressure on one's arms/hands - hard braking on a modern sport bike is going to put weight on your arms. Trying to turn a bike at 100+ miles an hour, takes a fair bit of strength (for example, even at my pace this last round I was really struggling to turn the bike in the fast 8s - just a lack of power on my part from not working out enough, but that directly relates to pressure going into your hands).
I haven't noticed a huge difference between the different grips, I think my pace has a bigger effect than anything. Trackdays, street, no problems. Start pushing up to a race pace and I start to have issues.