Yeah, well Messiaen didn't exactly have an orchestra available to compose for in Stalag VIII-A. :shifty:
Seriously, the instrumentation was entirely dictated by what was available. Messiaen didn't have a plan from the start, he was just adjusting as the situation improved. For instance, the third movement, for solo clarinet, was written first because Messiaen had a clarinet (and a clarinetist) handy.. he didn't even really plan to use it in a larger work at that point.