Hi TotalBeginner,
Mach Number. See:
Aero 12 - Angle of Attack about half way down,
"Because of the effect of Mach number on stall AOA, the stall warning AOA typically was set at a conservative level to accommodate gross weight and altitude variations expected in the terminal area.
The early stall warning system thresholds were not set to be effective at cruise altitudes and speeds because they did not correct for Mach number (fig. 10). This kept the system simple. The stick shaker was set at an AOA effective for low altitudes but at too high a value for cruise. Natural stall buffet was found to give satisfactory warning at higher Mach numbers."