I've been away from the Warhammer world for some years now, so correct me if I make wrong assumptions. I think this question is on-topic because of the "behind the scenes" kind of question we allow here.
From the wikipedia page :
Warhammer: Age of Sigmar (AoS) is a tabletop wargame released by Games Workshop in 2015. It replaces Warhammer Fantasy Battle (WFB) as the fantasy wargame in the Warhammer universe. Several supplements released for the eighth edition of WFB centred on the End Times which led to the almost total destruction of the Warhammer world and the death of most of the world's population. This leads into the Age of Sigmar with the return of the long-lost founder of the human empire worshipped as a god.
Browsing the Games Workshop site it appears that the minis base can now be indifferently round or square. The rules appear to have been simplified a lot. There are only 4 factions (or Alliances), somehow divided into smaller branches.
I could go on about the changes because there are apparently a lot.
What is/are the out-of-universe reason(s) that pushed Games Workshop to go through such a rebuilding of the Warhammer world and rules ?