Presumably Docker offers some benefit. It's weird that you and several others have jumped to the conclusion that it's obvious they should get rid of Docker. I would say: knowing this information, why do you still think java is a good fit for their use-case?
a) Their service was already written in it
b) Their service was already performing adequately before they brought docker into play
But those aren't answers to your question. Java may have been a terrible fit for them, but their decision to move away from it was not motivated by that -- it was motivated by their already-written, already-performing Java code no longer performing as well, because of Docker.
The only question is, does Docker bring them enough benefits to justify reimplementing large amounts code, in a new and unfamiliar language? Maybe it does, but somehow I doubt it.
