The reason?
It was in a nutshell.

A book which sold at retail for one dollar and a half cost the bookseller ninety cents.

If it was at all a popular book, "Thurston's" advertised it at eighty-nine cents--and in any case at a profit of only two or three cents.

Of course it was done to widen the establishment's patronage--to bring people into the store.

Equally of course, it was destroying the book business and debauching the reading tastes of the community.