Its both the Google Play certification (which takes a minimum of 2 weeks), and the QA process of Cyanogen and OnePlus. Both Cyanogen and OnePlus have set QA processes (automated and manual). Execution of these without issues is 1 week. If QA is working, then that means there are failure points discovered, which need to be resolved and then tested again.
The short story of this is as such: on the open source/community side, restrictions and barriers to release are much less stringent than on a consumer facing device - you expect and should get better consistency in the releases.
