valve enabled publishers to lower their risk and rode the wave of best buy and others shrinking their PC game sections.
if you print a million retail copies of a game and they don't sell then the publisher has to take them back and eat the cost. steam eliminated that risk
and the profit margins on games are pretty low so most retail stores don't want to stock them or give them very little shelf space. steam solved that problem as well
the last was the rise of laptops over desktops. people going on trips and wanting to take their games with them with no disks. steam solved that problem as well.
now that all these problems have been solved the publishers are looking to cut costs even more and their own stores is the answer.
and one of Jack Welch's most famous quotes is "Don't let anything get between you and your customers." Just look at what happened last year when EA and MS went head to head about MMO fees for some game