Popularity I assume - people will be less hesitant to download an apk file from github, even if being on github doesn't really make it trustable in practical terms.
Well yes, they're called "private". Often either illegal, it offends some powerful people or the creator doesn't give a shit and may just post a link on a forum as a thanks.
Respect to those who put it out in the open! A DMCA is nothing, but people have died and even went to prison for a cool gray area program.
One of the reasons why the original vanced was taken down was because it was advertised way too frequently and publicly. These projects should always be disseminated by word of mouth (hn/reddit does not count)