MongoDB changed its license to be untenably restrictive.

It’s been more than a year when the upstream MongoDB decided to change the license of the Server code. The previous license was GNU Affero General Public License v3 (AGPLv3). However, upstream wrote a new license designed to make companies running MongoDB as a service contribute back to the community. The new license is called Server Side

See here: http://r-pkgs.had.co.nz/tests.html