There is a difference between a hobbyist making maps and a developer making maps.
The developer can’t just draw a quick plan and start making the map, he has to go through a lengthy design process planning out the layout and how the objectives will play out. As it’s an official map it will have to tie in with the story meaning the writers have to get involved and cutscenes with voice actors have to be used like all the other maps.
They most likely have some ‘rules’ on how a level should look like to keep consistency with the aesthetics of current maps but different enough so it stands out on it’s own as such lots of new assets would need to be created as re-using too many stock assets would feel cheap. They also did a lot of research for the ark which they would need to keep referring to in order to make sure it makes sense in the context of the universe they have created in Brink.
Then the map has to go through internal testing and balancing then passed through Bethesda QA (assuming such a thing exists) and even then the new releases have to pass certification on the consoles. Also there would be deadlines to be kept and such meaning there maybe lots of overtimes and extra man power needed to make sure it reaches that deadline.
A custom mapper can just make a map however he sees fit and take as long as he wants on it.