Part of what makes Market Garden so perplexing is that it was so out of character for Monty
My reading is that Monty was responding to earlier pressures from London to drive the pace. On top of the well-known issues of manpower shortage and the need to show British ‘pluck’ to keep spirits up on the home front, there was concern over Stalin’s intent. By this stage of proceedings, Churchill had identified the potential for post-war tension and a strategy of beating the Soviets to Berlin had been discussed in London. (The same logic explains Churchill’s seeming obsession with the Mediterranean theatre long after it ceased to be critical to the Nazis’ defeat).
The Americans had other strategic concerns. So when Monty agitated for support from London for his ‘rapid thrust’ idea, which he thought was aligned to Churchill’s intent, he was surprised by the tepid response. The political realities of coalition-keeping had taken hold without Monty being aware. Churchill had gone to view the Mediterranean landings in person and Brooke was left to break it to Monty that he would have to play nicely under Ike’s Alliance-friendly ‘broad front’ strategy. Logistics were insufficient for two rapid thrusts across Europe, while a single thrust would have to be led either by the British or the Americans, which neither would countenance.
Eisenhower approved Market Garden essentially as a sop to Monty: a chance to begin a ‘single thrust’ approach without risking everything and to see what came of it. Even this limited ambition outraged Patton, desperate for fuel to continue eastward, but Bradley had the good sense to recognise that he had to cooperate so that Monty (and by extension the British public) couldn’t blame the US for any failure. And that calculation just about worked. As an exercise in leadership and coalition management by Eisenhower, it was expensive but effective. The mind boggles as to the scale and impact of the decisions he had to take.