Today, Drupal 8.1 has been released and it includes BigPipe as an experimental module.
Six months ago, on the day of the release of Drupal 8, the BigPipe contrib module was released.
So BigPipe was first prototyped in contrib, then moved into core as an experimental module.
Experimental module?
Quoting d.o/core/experimental:
Experimental modules allow core contributors to iterate quickly on functionality that may be supported in an upcoming minor release and receive feedback, without needing to conform to the rigorous requirements for production versions of Drupal core.
…
Experimental modules allow site builders and contributed project authors to test out functionality that might eventually be included as a stable part of Drupal core.
With your help (in other words: by testing), we can help BigPipe “graduate” as a stable module in Drupal 8.2. This is the sort of module that needs wider testing because it changes how pages are delivered, so before it can be considered stable, it must be tested in as many circumstances as possible, including the most exotic ones.