None of the above answers actually answer your question. Lead was historically used in gasoline for two reasons - 1) to increase the octane rating (a measure of knock resistance) ans 2) to lubricate the valve seats in the cylinder head. There are multiple other chemical additives that can accomplish #1 (the fuel in the article uses one of them). For #2, the solution in the automotive world starting in the 1970s was to use hardened valve seats that didn't require lead in the fuel to lubricate. The actual answer to your question is "because the FAA won't allow it".
