In the fourth episode of Band of Brothers, Nixon, the intelligence officer for Easy Company, states that the Germans had only left "old men and boys" to defend Holland. If this is a fair characterisation of Allied intelligence, how was it that an operation committing such enormous resources rested entirely on such grossly inaccurate intel? Having reconnaissance planes flying over Holland would clearly tip the Germans off, but surely the Dutch Resistance were only too aware of German power in the region, and channels could be found for them to pass accurate information through?
This subject (along with the general lessons of Market Garden) was something I spent a few weeks studying during my officer training, complete with tours of the battlefields in the company of a semi retired general and a few interviews with survivors.
There were a lot of factors involved, the allies very much did send reconnaissance planes but reports were often lost in the chain of command or simply took too long to be verified and passed on to senior commanders. The 9th and 10th SS panzer divisions were moved into the region only two weeks before the operation started as an example, not as part of any kind of defensive plan but in order to allow them to recuperate and reinforce in a safe area behind the lines. Their presence was absolutely key to the operation's failure and in a stroke of particularly bad luck for the allies, they had been specifically and extensively trained in anti airborne operations.
Also war is very much a 'two way street', some of the low level German commanders in the area around Arnhem were very good indeed, Sepp Krafft for example was in command of a training battalion and as soon as he was notified of the parachutes and gliders, roused his command then had dug into slit trenches in a perfect blocking position within an hour. It was literally the best possible response to the British landings and it was carried out in a blindingly short period of time despite virtually no knowledge of what was happening.
There was however a huge degree of redundancy built into the whole operation, keep in mind that it only barely failed despite virtually everything going wrong and there being two unexpected armoured divisions (albeit at low strength) in the region. Additionally one American officer carried the full plans for the entire battle on his person then died and had them retrieved by the Germans, which was not something he should have carried with him!
Also for anyone getting their information from the film, XXX Corps did not stop pushing for Arnhem from Nijmegen due to being too British and requiring tea. The road that had to be advanced down is raised above the surrounding terrain on an slight embankment with what is basically swamp to each side and there were anti tank guns positioned covering it.
Here is the road today:
It was narrower back in the 40's, only a single lane each way and with less well drained land to each side, an absolute nightmare to try to push an armoured corps down as the only line of advance.