Most scholarly journals that aren't owned by a large commercial publisher run on a complete publishing platform. Open Journal Systems is the industry leader;2 Janeway is a newer and genuinely well-made alternative.3 They do serious work, and for a university press running many titles they may be the right tool for them.
But they are complete systems. Each is a separate application — its own software stack, its own server, its own logins — that someone has to install, secure, update and keep alive. For many publishers OJS arguably does too much, and because there isn't much of a market for journal software, there's comparatively little public knowledge about how to run, theme or fix it. The handful of lighter, plugin-based options that have appeared over the years have mostly gone unmaintained.
There's also a quieter mismatch. A learned society, an association, a department, or a research centre rarely starts from nothing: it already has a website — and that website is, more often than not, WordPress.4 Running the journal on a separate platform means a second site beside the first, with another system to learn, separate accounts, and one more thing that can break — when the place a journal most naturally belongs is the website you already have.