Maybe a real reason is that it's precisely a distro which means it contains features and reasonable defaults so that users don't need to learn about them, install them, and configure them.
Basically same debate as between Linux From Scratch and a full-featured distro such as Ubuntu.
* Alerting - you can use ElasticAlert
* Security - Search Guard
* SQL - https://github.com/NLPchina/elasticsearch-sql
There very little reason (IMO) for users to a choose "Open Distro" except that it comes as an AWS image.