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Heat. Lots of it. Old-school turbocharged gasoline engines use a lot of gasoline to cool down the intake charge and combustion chambers in order to prevent detonation. In fact, many of them run the risk of incurring detonation due to running way too rich.
It was simply easier at the time to do it this way than to properly tune the engine to prevent grenading. Sending your car to a market that has potentially bad gasoline? Turn the fuel taps up. Turn them waaaaay up. I once saw a naturally aspirated Toyota on the dyno hit 10:1... at idle.
With a better intercooler and more predictable spark, you can lean that out pretty easily. With modern engines running direct injection, you can lean it out a whole lot more.
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