A difficult question, some flowers do last a lot longer than others, for no apparent reason. In the case of orchids commonly grown for cut flowers for the home, i would hazard a guess and say, its the tough waxy protection that helps in its long lasting properties. most orchids for home use have evolved in a warm, moist atmosphere of cloud/rain forest and often grow high up in trees as epyphyts (seni paracitic) It probably this type of environment that the plant has evolved its waxy coat to shed exess water from leaves/stems down to the roots. Another factor could be that its evolved to be pollenated by specialist insect pollenaters for its reproduction and would therefore maximise its chances by displaying its flowers during the adult stage of the insect etc. Well thats my theory anyway..