I'm not sure, but Rock Shox used to sell 1" steerer tubes as replacements on some forks. It might be worth asking your LBS if it's easier to change the steerer on your new fork to match your frame.
As for your question on what brought about this change in headset size? In about 1990 Gary Fisher came up with a 1-1/4" Evolution headset, and other makers followed suit with 1-1/8". The larger size has been working it's way into all mountain bikes and most road bikes since that time.
I've never felt any difference in front end stiffness with any size, and I've owned all three sizes.