K8S is developed by a multitude of very large companies. Each with their own agenta/needs. All of them have to be addressed. Thus the complexity. If you think about it they probably manage to keep the complexity to relatively low levels. Maybe because it is pushed to the rest of the ecosystem (see service meshes for example).
Being pushed by the behemoths also explains the popularity. Smaller companies and workers feel that this is a safe investment in terms of money and time familiarizing with the tech stack so they jump on. And the loop goes on.
Main business reason for all that though I think it's the need of Google et all to compete with AWS creating a cloud platform that comes to be a standard and belongs to no-one really. In this sense it is a much better, versatile and open ended openstack attempt.