That’s because Montgomery was in charge.
Montgomery had actually proposed a concentrated Patton style drive straight for Berlin arguing that it would bring a quick end to the war. But like Patton, Montgomery wasn’t considering the huge logistics problem it would create. Patton’s own army eventually stalled because he outran the ability to keep it supplied with gasoline and ammunition. A lot of strategic planning comes down to logistics.
Eisenhower struck down Montgomery’s idea of going straight for Berlin both on logistics and the fact that it would create a 500 mile long flank that the Germans could attack at will. Market Garden was a very limited replacement aimed at keeping the German army in the north under pressure.