It may surprise you Geezer, but not everyone is Computer literate, especially older people. The paper version is still very profitable and widely used. Advertising in the Yellow pages is pretty expensive, but still has a proven record of working despite the competition from tinternet.
No company as big as Yellow Pages is going to jack in a successful business to save a few trees.
These directories are made from recycled paper and most councils will now accept the old directories for recycling, so it is no worse than buying a daily newspaper in terms of damage to the environment
(Sorry went into huge beardie mode there).
http://www.recyclenow.com/what_can_i_do_today/ can_it_be_recycled/paper_products/yellow_pages .html