Total War: Warhammer makes a bold departure from the former franchise's classic gameplay to honour the latter property's lore.
In Total War: Warhammer, commanders can't conquer every region on the map. Which regions can be occupied, as opposed to raided or destroyed, depends on your race and that of your target.
There are two kinds of regions. Holds can be occupied by Dwarfs and Greenskins, while Human Settlements can be occupied by everyone else. The only exceptions to these rules are the Chaos Warriors, who do not settle anywhere, and the regions in Norsca, which can't be occupied by anyone.
By making this decision The Creative Assembly has not only held true to Warhammer lore (it doesn't make sense for humans to occupy an underground dwarf stronghold, for example, and it has practically never happened in the franchise's canon) but made some interesting gameplay decisions, too.
In a blog post on the Total War Wiki, The Creative Assembly community coordinator Joey Berry argued that the vast majority of campaigns are over long before every region on the map can be taken, and that victory by expansion is the least strategically interesting way to play. Map design has been carefully balanced to ensure players have more than one route of expansion, so they won't be completely stuck if one avenue closes through good luck.
"We are not just aiming to put a Warhammer hat on Total War," Berry added.
"We have the opportunity to make something challenging, new and different from what you’ve played before, all while still having that crucial TW feel."
Future Total War games won't necessarily continue this style of play, as human on human conflict doesn't inspire the same limitations.
It's really worthwhile reading the full blog post as it goes into a lot more detail and provides example scenarios to illustrate the Creative Assembly's thinking. Hit the link above for more.