Only my opinion but the media, especially movies, played a part. I recall the HOB series "Deadwood" showed Wild Bill playing poker with what appeared to be Ivory chips (a leaf as a $1 chip, $5s and $25s all white chips). If you just followed the old westerns, every gun had Ivory or pearl handle grips on their guns.

I have Ivory set from an old west saloon:

http://www.oldpokerchips.com/IVORY/DanielClay.htm

There are the El D'O (El Dorado) chips. Personally I'm not sure about them.

If I had to guess, I would think a good percentage of mid to upper class western saloons used Ivory chips. Having said that, I have ivory chips from the Madison Square Club in NYC.

The vast majority of chip carvers we know of were located in NYC (many where the WTC is located). Some in San Francisco.