Oh my! I wonder if the competition of all the online game sites and wiki's is the reason.
I think there's more to it than that.
Walkthrough sites have been around since the late 1990's.
That didn't stop people from buying the nice, glossy, book versions of Guides for their very favorite games.
I think the biggest problem is that the current top-selling games are online games that keep updating, adding features, removing features, rebalancing gameplay, etc. Printed-on-paper guides can't keep up with constant tweaks to the games and can't stay accurate.
eBook versions could stay up to date, but there are already websites selling nicely formatted, professional-looking, color pdf guides. So even if there weren't plain text walkthroughs available for free, there is already competition from websites selling eBook guides.
There's also an argument that many of the people who liked the Guides liked "hard copy" in general -- and most games aren't sold hard copy anymore -- so those people who wanted "hard copy" tended to stop buying games and no longer need Guides for them.