Or is it just that more knowledge about maintaining coverstocks is available?  I don't think the idea of using a low grit followed by a high grit was that well-known then.  Going from a low to high with many intermediate steps in between was more common then.

I have a couple of balls from around 2000 that I still use on a semi-regular basis and they still seem to have the same hook (really difficult for me to tell because the house has changed patterns numerous times and I have made changes to my game too).

It seems today (to 15 years ago) that ball maintenance is at a all time high. I believe there are some coverstocks requiring maintenance after 10 games. If you want high performance you are going to pay for it. 

It's kind of funny that it has completely gone in reverse compared to the old days of bowling. Back then you wanted that old beat up ball.