Not to split hairs (^^) but:
It sold for €3,175,400 not $3M
That's not a question of liking the background or not. The editor thought that this cover was too complex and would be too expensive to print, that's why the definitive version is much simpler.
It was a prelim, even if it looks like a finished work. You can see that the face of Tintin is not finished, for instance. Plus Hergé fold the illustration and stapled it to the letter sent to the editor (we can still see the small holes). Who would do that to a final cover?