The reason is much like why we fly the airplane using airspeed rather than ground speed. The things that we are concerned with as pilots, like when the wing stalls in level unaccelerated flight (I know it's angle of attack but work with me here), the best speed for approach, best cruise speed etc, are all functions of airspeed. At higher altitudes and speeds, things like the speed for high speed buffet, low speed buffet, minimum drag speed, best range speed etc, are most easily described as functions of Mach number. There are other variables as well (weight and temperature) but using the ratio of true airspeed to the speed of sound simplifies things somewhat. You could use indicated airspeed but the values that you would be concerned with would be all over the place as weight, altitude and temperature changed.
In a nutshell, the phenomena that you are most concerned with as a pilot (or performance engineer for that matter) are most easily described as a function of Mach number at higher altitudes.
Last edited by Gillegan; 27th Aug 2015 at 13:43.