User blog comment:Rupert Giles/Introducing Updated Global Navigation/@comment-28736347-20141107223913/@comment-2.96.27.188-20141130164139

Notice that all the hubs excluding lifestyle are based on media. Media sells adverts. History and science, no matter how many people find those topics interesting, do not have nearly as many advertising or sponsership oppertunities. Lifestyle is merely a "catch all" for a large chunk of wikis which don't fit in the other six. I suspect advertising oppertunities were the main factor behind the choice of hubs.