The idea is to have many mirrors of the Tor website across the world, with some of them on websites that governments will hesitate to completely block. These include Dropbox, Github, Google Drive, Amazon S3, the Chrome app store, and so on. Because of the "https" encryption that these sites use, it's hard for the censor to figure out which particular page you're loading from a given site, so they have to choose between letting everything through or blocking the whole site.
Of course, it’s not always so simple -- for example, the censors could pressure each site one at a time to take down the Tor files. Getting your copy of Tor from a friend that you trust is still a great distribution approach.