There are a number of perfectly understandable reasons for all this.
1. Cumbersome purchasing procedures. By the time the specification has been written, the requirements have changed.
2. Internal politics - the guys that know whats what don't get to make the decisions, as well as cost pressures.
3. Lack of engineering skills - like the ^&**%$ moron who specified flexible fuel injection pipework for the Westralia.
4. Careerism, as in "Would I like to go to Switzerland to check out PC9's or the Wamira at Fishermans Bend in Melbourne?"
Sorry, I think I've forgotten to take my anti cynicism pill this morning.....