The very purpose of copyright, at least for publishers (then called
stationers - early 18th century), was precisely to get a monopoly on
the publishing of a work, so as to improve their return on investment.
You do not have that monopoly with the public domain.
That being said, it is not completely true that public domain is uninteresting for them, depending on the
competitiveness of the market structures. Foreign books were not
protected by copyright in the USA for most of the nineteenth century. Mark Twain
fought valiantly to defend the copyrights of British authors ... so
that they would not be unfair competition to his own works.
It is also true that big publishers still publish a lot of public domain
books, but only for very popular works which they are sure to sell.
Small publishing houses may publish more specialized public domain
works for specific audience. It is a very honorable activity, but they
do not get very rich.
But in the digital world, there is no longer that much investment to
produce copies of a book (copyright is about making
copies). Publishers were acting pretty much as banker who invested
money up front to produce a number of copies (so that each copy would
not be too expensive), and recouped the investment as copies were
being sold, if they were. This is no longer needed in the digital world,
and publishers have lost their main usefulness. They had other roles
that can still be provided as service, not as an industry controling
the whole business of textual publishing.
I suspect publishers will cease to exist as we know them in a not very
distant future. Not needed economically.