Pressures of war?
I cannot give a definitive answer to that question but Power Jets was operating on a bare minimum of funds, they couldn't possibly arrange the acquisition and modification of an aircraft for that purpose. From the Air Ministry's point of view this engine was something entirely new, necessitating a new airframe to show what the engine could do. Also modifying a bomber or transport would take an operational aircraft away from the front line. I guess that in peacetime the cautious approach would have been used, but with the war they needed to move forward with the program.