I think this is because the Mesos project has not yet added their DOAP to the list
---------- Forwarded message ---------- From: Vance Cagle <[email protected]> Date: 10 April 2014 16:49 Subject: Mesos Missing from Product List To: "[email protected]" <[email protected]> Hello Apache.org Webmaster, I thought you should know that Apache Mesos is missing from the alphabetical projects list at http://projects.apache.org/indexes/alpha.html#M
