Because Mexico City with its thin air, at 2285 meters above sea level, the F1 cars are reaching 370 KPH or more down the main straight, and if I not wrong, here is where the F1 cars are reaching the highest speed of the whole calendar.
But apparently the gearing was not the cause of the lack of top speed in Monza and is more Honda setup related, so this is already solve, per Jos Verstappen comments.