For development: different reasons, depending on your use case. You have to always keep in mind who is using the solution in the end. If it's a client, it often means that the executive or marketing team will use it. "Headless CMS" or "regular CMS" means nothing to them and it shouldn't either.
(1) I've worked with a variety of open source CMSs (mostly PHP based), some are overly complicated to learn for someone without serious coding knowledge, some have been abandoned, leaving clients and users in the air, and some have changed their business model, forcing adjustments on my side. Wordpress was easy to learn (I've never had formal training, so it was all self-taught), it had very good documentation for a newbie, a huge community, it remained free and open source and it kept maturing. Installing and configuring WP is notoriously easy and the server requirements are almost never an issue. WP has been reliable all these years and I can't say I trust many of the emerging platforms to match it: the last thing I want is to go full in with a new tool which will be dropped in a few years because the creators couldn't monetise it or find the time to maintain it.
An easy choice for me and others like me.
(2) I've had to build websites for clients both alone as part of a team. Apart from the extremely rare case where a headless CMS was explicitly needed or requested by a client, regular WP was perfectly fine to do what they needed a CMS to do. Furthermore, many explicitly requested WP or a similarily popular open source CMS, in case they needed to work with a different Dev. A sensible business decision, in my view.
(3) Huge pool of developers, very large community, generous amount of tutorials on various aspects of customisation, incredible number of extensions and themes.
For content writing & management: different reasons again, depending on your use case.
(1) For my own projects, I see little reason to mess about with shiny new CMSs which force me to learn new unnecessary things or change the way I already do things after I've used a platform that simply works for about 11 years. This thing does the job well, it's as simple as that.
(2) It's genuinely easy to use for most people. Now I know this is a subjective appreciation to a good degree, but while I'm not saying that it can't be improved, I want to stress the fact that most clients who've use it for a while learn its logic rather fast and become independent users (in terms of publishing content) in a short amount of time, compared to other CMSs.
(3) 90% of our clients are not technical people, and even some of those that are technical don't know much about CMSs or don't care beyond some specific needs. Speaking to them about JAMStack and headless CMSs is like a car salesman who goes so deep into engineering and materials composition that scares the clients away. Instead, we ask what is it they're trying to achieve with their websites and what they think it's best way to do it; Wordpress almost invariably passes the test when we present it to them.
People who write & manage content are generally marketing people and execs. They care more about design and SEO than the technology itself, which makes sense.