Luftwaffe test pilot Flugkapitän Erich Karl Warsitz (seen here on 1950). On 27 August 1939 he flew the Heinkel He 178 proof-of-concept prototype jet aircraft. He also flew the first rocket-plane.
Same as in the Royal Airforce - Often it was just too expnsive! Or someone couldn´t stand the change in history/military tactics etc.! Contracts were made for other types of piston fighters etc. and so that contracts had to get fulfilled. The common madness.