The simple answer is “because DocBook V5.0 is the future.” Apart from this marketing blurb, there are also more technical reasons:
DocBook V4.5 is the last version of DocBook in the 4.x series. Any new DocBook development, like the addition of new elements, will be done in DocBook V5.0. It is only a matter of time before new elements are added into DocBook V5.0, but they are not likely to be back-ported into DocBook 4.x. DocBook 4.x will be in maintenance mode and errata will be published if necessary.
DocBook V5.0 provides significant
improvements over DocBook 4.x. For example, there
is general markup for annotations, a new and more flexible system
for linking, and unified markup for information sections using the
RELAX NG offers many powerful constructs that make customization much easier than it would be using a DTD (see Chapter 5).