First of all, military operation is always a part of politics. Therefore, civilian leaders had a final say in the decision making, especially in the grand themes. At the time, US army has not proven itself in the battle, at least not considered as battle seasoned as the British army.
In North Africa, Monty (Field Marshall Montgomery) orchestrated the tremendous victory at the el Alamein Campaign and decimated Nazi African Korps. US Army merely entered the scene then. Naturally, Monty's opinion carried more weight at the time leading to Market Garden. Ike (Eisenhower) as the supreme commander may have raised his concern but obviously not enough to pull back Monty's plan.
Operation Market Garden was a strategic brilliance however a tactical disaster. If successful, it will greatly reduce Germany's war capability by threatening the industry land of Ruhr, and may have ended the war before the winter came or if not, at least prevent subsequently the Battle of Bulge. (in which US Army was battle hardened and emerged as the main thrusting force overwhelming the German.) Ike wanted to concentrated his supply and effort further south in France and then cross the Rhine into southern Germany in the US zone. But he also had to yield in the political game within the Allied circus (at the expense of lives of ordinary soldiers and civilians, but history can not be experimented or relived, nobody can say for sure that Monty would fail).
After Market Garden, Monty lost his halo and Ike was much freer to command. Naturally, the Ike-Bradley-Patton line prevailed. If you are into this, I would also suggest you look into the CBC mini series "Dieppe". It was a sacrificed attempt mainly suffered by the Canadians, which I think basically paid its due to the Queen.