Things aren't that quick to change. Remember, cells are still mostly acting normally in the body during a hypoglycemic unconscious episode. The person likely overdosed on insulin (or didn't eat), so their blood glucose level is low, but think about where that glucose went. Insulin is a hormone that activates glucose transporters into the cells from the blood, so intracellular glucose is normal/high and the cells are ticking along through cellular respiration just fine. We just don't measure intracellular glucose levels.
The unconsciousness is just because the brain requires a continuous supply of glucose directly and is very sensitive to rapid changes in BGL. That's also why it's so quick to fix though after a bit of D50.