Learning how to use a content manager depends a lot on the approach of its developers. Until a few years ago the CMS were focused on companies that were going to serve customers, and that is why the learning curve of the CMS was usually high, very thought about programmers.
This was very logical, since there were many people living from that and almost no one was considering – or knew it was possible – to personally manage their website or their company's, these things were delegated to computer consultants or web programmers.
But the thing changed little by little …
My first standard CMS (I tried other made-to-measure ones before) was PHP Nuke and I still remember the sufferings I went through to perform simple tasks, let alone manage it.
Later, with Mambo – later Joomla – the sky opened and I perceived what the future of the CMS should be. However, the content managers were still too oriented to programmers and webmasters, doing complicated tasks that I thought should be simple.
I usually comment that the evolution of CMS is similar to the history of computing which was first intended for companies until a company called Apple thought that those computers should be user-oriented and designed graphical interfaces that still today remain a standard of usability.  With the CMS the same thing happened and although Joomla improved a lot in terms of publication facilities for publishers, I still had hindrances in terms of file management, let alone the administration, which was quite cumbersome.
Drupal is similar but even more thought of in developers. The first time I used it I was pleasantly surprised by the simplicity of its administration compared to Joomla, but it suffered from many shortcomings, especially for the novice user. After many sleepless nights reviewing documentation I decided that I had to use my life for something more practical.
And here came WordPress that little manager thought about blogs, that is, about people, that little little was increasing its possibilities, but without ever losing the original focus: oriented to users not to programmers .
And I think that is the fruit of its success, not having lost the original orientation, and increasing its functionalities, becoming a powerful and adaptable CMS surpassing the other content managers, but without losing the point of view that it should be oriented to any type of user .
That is why today the WordPress learning curve is the least, which requires less effort from the user . And does this mean that it is not powerful enough for the developer? Well, neither does it, quite the contrary. Its well-organized structure, ease of programming using hooks and functions gives a freedom, flexibility and scalability to the developer currently unsurpassed in the world of CMS.
And of course there are WordPress courses but what they usually offer are even faster, expert-oriented learning paths to adapt all the knowledge and tricks of the WordPress ecosystem to the needs of the individual , not because it's difficult.
That's why, just for that, WordPress is the king of CMS for a long time while the rest do not change their focus, and look towards the user.