Why use DocBook when there is HTML?

Mark Pilgrim abandoned DocBook in favour of HTML for "Dive Into Python 3".
See
http://diveintomark.org/archives/2009/03/27/dive-into-history-2009-edition/.

The author used DocBook for the previous edition of his successful book. But
now he says that he has no use for DocBook anymore.

What are your thoughts about this? 

Mauritz