Joomla is a free and open source CMS that's been around since some time. They have a reputation of being user friendly and popular at the same time. It is not hard to ask someone in the web development field about Joomla and not to get help. People would have tried it at some point of their career. Joomla a great ecosystem around it. Themes, frameworks, plugins, modules, addons and a whole lot of things you can add on top on the core CMS
Joomla says it's SEO ready. I don't see why they are not considering that it is one the CMS that Google might actually know about and react to it. Joomla has a very helpful community around them who will help you out when you are in trouble. You don't have to pay for it as well. Apart from official Joomla community, there are a lot of places where you can get help for Joomla
The above link is an official Joomla resource for training in Joomla
Sadly, Joomla comes only second to WordPress which runs around half the websites on the web. That is reason for it. WordPress is more straightforward than Joomla in the back-end. But that said, I believe Joomla can do more than WordPress when it comes to functionality.