The reason why German mustard is so good is because German mustard uses whole mustard seeds and often with a blend of yellow and brown and a variety of spices and aromatics that give the German mustard a bold and complex flavor profile.
German mustard also sometimes includes additional ingredients like honey or horseradish which also helps improve the flavor.
The addition of honey or horseradish in the German mustard results in a depth of flavor which sets it apart from other mustard's.
Most American mustard's use powdered mustard for the mustard seeds and German Mustard uses the whole mustard seeds.
German mustard also can contain onions, allspice, cloves and more to add flavor.
Also different regions of Germany have their own unique styles of making mustard and some like Bavarian mustard's that are known for their sweeter profiles that are achieved through addition of adding sugar, applesauce and honey.
German mustard's come in a variety of different flavors and texture which include mild to spicy mustard's as well as sweet and savory mustard's.
Mustard also has a long history in Germany and dates back to medieval times where the mustard was also used for food flavoring for foods and medicinal properties.
German mustard is also darker and has a more subtle taste than English or American mustard and English mustard is yellower and is much stronger.