The operation occurred only months after D-Day, so I assume that the Germans would not have had more forces there pre-D-Day.
Because Holland is an extremely difficult place to make a major seaborne operation. It doesn't have beaches that could be used to land 6+ divisions in a single day. The poulders (low-lying fields) could easily be flooded to block invading armies. The bridgehead could be isolated by keeping or destroying bridges like Arnhem. The Netherlands were close to German U-Boat bases in Germany, which would threaten the invasion fleet.