Thing with Airfix is they've just got to provide the sprues full of parts, no assembly, no painting, adding details, no delicate packaging. Bag the sprues, drop in the printed instructions and put the lid on the box. Job done! Keeping the popular lines stocked up on the shelves is easy.
Hornby have a more complicated production process, resulting in an RTR model.
As for the B4, I don't think it was ever intended for production and the Nellie, Connie and Polly locos are traversties of the originals. I expect a green no 27 will pop up sooner or later!