It's just a function of cost. There's several racing fuels that have R+M/2 octane ratings pretty high, up over 100 with no lead.
http://www.vpracingfuels.com/c20-spec-sheet
But it's very costly, and adding lead stabilizes the fuel, maintains good RVP, and allows the additives for high octane to work more efficiently.
Unleaded replacements for avgas can be made, and have been made. The crux of the problems right now are primarily political. The FAA doesn't want a single-source solution, and they are scared of any small refiner(not Shell, Phillips, Sunoco, etc) having a lock on the unleaded market.
Once one or more of the big boys makes an effort, the EPA will get busy and ban all leaded fuels, we'll all switch over, and scream about the price for a while, then it'll be business as usual with about a 10-30% increase in cost. Those who are smart will invest in planes that are STCed for unleaded mogas. Anyone with a turbo - is screwed.