Good insight, its true, you would think if they were available 2 years ago, they would have been available 20 years ago.
I think there are a few possible answers. One is that 20 years ago, someone knew they could get away with it, one was on backorder, so they just go the car out the door faster.
Another possible answer is that I know that sometimes I have had to wait for parts because they are waiting to do a run of said parts. So my options are to wait, or install what's available. That decision should be made by the customer, but sometimes it is made by the installer if they feel there is no difference except getting your car back to you sooner. Maybe 20 years ago they were out, then someone found old stock, or a new run was done, then 2 years ago they were both available. I looked six months ago and could only find the one side. Who knows if it will change again in the future!
I believe, as Jonny mentioned, they had part numbers for bad roads, and normal use, then an updated one. Perhaps they just found a new material that did the job of both.