Why are paperback O’Reilly books never printed in color?

I regularly buy technical books from O’Reilly for which I pay anywhere between 30-60 euros. The print quality is ok (not great I would say) and the images are always back and white. I recently got a free pdf o’reilly book and the images where in color there. I found this to be a big improvement. Some images are much clearer and it gives the book a more premium feel.

Why are the print versions never in color? I usually pay between 40-60euros for these books which I don’t consider cheap. I assume the majority of the price of these books goes to covering fixed costs and that the cost to print one book is only a few euros. Why not spend a few euros more to get a more premium product while barely increasing the price?