My outsider perspective is that DeepMind was the research arm and Brain was specifically tasked with making the company money through AI/ML applications. This appears to me that Google is combining the two to make sure that DeepMind starts turning a profit by adopting Brain's mission. Of course, DeepMind's brand is orders of magnitude more valuable so it makes sense to keep the name around. Would be happy to hear more knowledgeable takes on if this is an incorrect reading of the tea leaves.