Pure speculation on my part but folks like the feel of cork. Doesn't get slippery ( Unless it is really dirty) transmits the feel of the rod nicely, resilient enough to be easy on the hand, and it is traditional.
Nothing else feels like cork, and lots of stuff has been tried. I have some spin rods w/ foam and have fished fly rods w/ foam and it is not bad, but it is not cork.
Bamboo builders can't help but be aware that their rods will outlive them. Lots of cork grips still look good after 100 years of use. Few plastics can hope to match that.
AgMD