That may well be because how the thing looked was important in the days of the early diesels, and thought was given to it,s appearence.
As time went on it became less and less of an issue it seems.
Which would certainly explain ugly locos like Class 67, and the hideous monstsrosity that it is about to be unleashed on what remains of our railway.
I think Freightliner should have their operators licence withdrawn for even considering allowing such a nightmare loose!

The cutaway model of the Deltic engine in the main hall at the NRM at York is fasinating and well worth studying - very clever stuff!