first of all, nitro engines run at around 35,000 - 45,000 rpm, where as gasoline engines run at less than 1/4 of that speed.
and secondly, it would be difficult to make a gasoline engine small enough for 1/10 and 1/8 cars, and even if they did it would be much heavier than a nitro engine.
when you miniaturize something, there comes a point where going any further would make it not work effectively or efficiently.
Hope this helps
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