Apple sold keyboards with a Danish layout before the standard was created and published. As we can only guess as to why Apple's engineers chose to do what they did, it is reasonable to guess that this history has something to do with it. I.e. why annoy existing users with moving about characters on the keyboard in order to follow relatively non-significant ISO standard. That said, as far as I remember, the original ISO7IEC 9995 standard didn't even specify the placement of the @ character on the keyboard. That was added to the standard decades later.
Note also that historically keyboards were very different from computer to computer. When you look at keyboards from the 70's and 80's, you won't find that they all featured essentially the same character placement - like you would find if you compared a 2000's or 2010's keyboard to a current keyboard.
So essentially, you're kinda incorrect in saying that "they moved" the character to that location - for Danes it didn't have a specific location where user expected it to be at the time. It wasn't like the @ character was placed on the same key as on modern Danish PC keyboards and stayed there for a few years before Apple decided to move it to a different location.
On the original Macintosh keyboard - the @ character even on US keyboards was placed on the first key in the top row (left of the 1 key). On the Danish version of the Apple II (europlus), the @ character was actually placed on top of the "P" key.
For comparison, a non-Mac computer that many Danish people got to know in the 80's were the Danish built Piccolo and Piccoline models. The Piccolo didn't have a @ on its keyboard, and the Piccoline placed it just left of the backspace key on the far right of the main keyboard.
Also you need to consider that at the time these keyboards were first introduced, the @ character was simply not used as much as it is today. Today most people know this character as the seperator in email addresses, calling it the "at character", etc. That was not the case in the early days of computing. At that time it was known as the "master space"* character, and it wasn't something a normal Mac user would need multiple times during a normal working hour. Hence when locations were scarce on the Danish keyboard layout, it probably weren't a huge priority to give it an easy to reach placement.
UPDATE: The question was changed to be about remapping the key. You can use the tool Ukelele to remap the location of the @ character on the keyboard.
*: Separator between commands and data.
