I believe it is customary for the estate to go to the eldest son to avoid diluting the value of the estate. Usually with instructions about his siblings. The second son usually goes into the military and if there is a third son he goes into the clergy.
It is traditional and old fashioned but Queen Elizabeth was a traditional woman,
I also believe that, maybe until recently, Andrew was probably her favorite. It is unlikely that she made no provision for him in her will.