Everything had to go right for it to succeed. That's always a recipe for failure, especially in war. You should always plan for things to go wrong or unexpected things to happen. The Allied leadership failed to do so.
The weather wasn't good after the first couple of days, so the Allies couldn't use their airpower to full effect. There was also a shortage of transport aircraft, so all of the troops couldn't be landed on D-Day (September 17th). The weather and the transport shortage made it harder to take initial objectives quickly and resupply those troops already on the ground.
The biggest failure was of leadership. Montgomery thought the area was lightly defended, when actually two SS panzer divisions had just moved into the area and he was told of that fact. The operation's ground commander, General Browning, was also warned about the SS divisions and didn't seem to take the danger seriously.
Because of these factors, plus the need to build several bridges (destroyed by the Germans) and a seeming lack of urgency on the part of the British XXX Corps (the ground force that was supposed to link up with the paratroopers and drive on to Arnhem), the plan fell badly behind schedule from the beginning.
There were other issues, such as the drop zone for the British 1st Airborne Division being too far from the objective (forcing a long approach march), but in the end overconfidence and bad leadership would in themselves have been enough reason for the failure.