Because Dutch is an adjective relating to The Netherlands, and Holland is only part of The Netherlands - Holland is a region in the western part of the Netherlands.
People call the country Holland, but they are wrong. There is a North AND South Holland and that forms the country of the Netherlands. The team name is correct.
It's like someone using the United Kingdom when they really mean England.
The north and south holland are merely provinces in Holland. The netherlands is the posh name to make people feel good. A lot of older philips appliances and children's books will have printed or made in Holland on it, not the netherlands.
The netherlands is a descriptive name meaning 'lower' (beneath sealevel) land.
and Dutch is related to Deutsch - the German word for German. Borders, and people, weren't always as clearly located as they are now. The Dutch sometimes refer to their country as Holland, so it's understandable if others do, but strictly it's the Netherlands.