In short, Docker's development was payed for by a company for commercial purposes. Moreover, it was build as an abstraction over kernel features so that developers won't need learn anything about them. It's success is product of the fact that tools can create extremely useful abstractions and when they do people benefit from using them and depends on them.
Docker was developed because a cloud provider (Dotcloud) wanted a better way to package their own and their customers' software. As it turned out, Docker was succesful while Dotcloud failed spectacularly. So Docker became the main product.. and now that failed too, as of a few months ago.