Historical accident, mostly. Sometimes there is a
reason; for example, GTK+ does not include gdk_imlib, so does not include any
widgets that rely on it. In very general terms, GTK+
imposes less "policy" than Gnome; some Gnome widgets
are deliberately inflexible to keep people from
creating an inconsistent user interface. GTK+ does not
take this approach. Finally, some of the Gnome widgets
were considered too "experimental" to go in GTK+ at the
time. However, the core Gnome widgets discussed in this
book are not in this category.
