Until someone more knowledgeable comes along...
The transonic region is the range of speeds from the speed at which the air flow first becomes supersonic over one part of the aircraft, to the speed at which all parts of the aircraft are bathed in supersonic flow. You know when air flow becomes supersonic at a particular location, because a shock wave forms. Since these shock waves form as a function of the speed of sound, people specify Mach number in this context. If there is no possibility of shocks forming (e.g. well below the transonic regime), there is little point in considering Mach number.
A shock wave is an extreme demonstration of air being compressible, but air is actually compressing and decompressing over a much wider range of speeds, without forming shocks.
Air is compressible at all times, but it's only significantly so above about 250kts TAS. So if for example you're going to measure the stall speed of an airliner at all altitudes, you will specify EAS for simplicity. It saves having to graph the change in stall speed (CAS) as altitude increases. If you specified it in terms of Mach number, you'd still have to graph the change in Mach as altitude increases. So there is no point in using Mach or CAS for a number that is essentially a constant in terms of EAS.
If you're cruising at maximum speed & altitude in an airliner, the potential for shock waves limits your performance envelope much more strongly than EAS limits performance. Hence airliners at altitude use Mach. Horses for courses as they say.
Hope that helps.
Last edited by Oktas8; 4th Sep 2012 at 11:49.