Purely speculation, but that could be because fatty acid is easier to store, so you need to use it later, and because glucose would be kept in reserve of high i

Purely speculation, but that could be because fatty acid is easier to store, so you need to use it later, and because glucose would be kept in reserve of high intensity effort (like running away from a predator) in case you really need it. Several fitness websites suggest that when exercising at 50% of your maximum heartrate, you burn more fat compared to carbs than when exercising at, say, 75% of your max heart rate. But that may be completely false, I don't know.