Popularity of Editorial Express
It seems that 77 journals use Editorial Express. Given that there are tens of thousands of scholarly journals, it does not seem to be immensely popular.
Anyway, one Editorial justifies the adoption of Editorial Express. It might answer parts of your question:

We have decided to use Editorial Express, developed by Professor John Rust at the University of Maryland. This software is very cost effective, and allows for much flexibility in how it is used. The software will send automatic reminders to reviewers, automatically save contact information provided by authors and reviewers, save all correspondence and manuscripts, and has many reporting functions. Editorial Express is Web-based, which allows the editors to use the software at any time, in any place. Ideally, this Web-based solution will allow us to coordinate the duties of John Mallinckrodt and myself, and might provide for some significant financial savings.

To examine the argument about cost-effectiveness further: The standard price of Editorial Express is $2000 per year, but can even be lower due to its &quot;lowest price guarantee&quot;-scheme. In comparison, one of its competitors, Manuscript Manager, has a graded pricing that may cost up to $8500 per year. Another one, ScholasticaHQ, offers a &quot;peer review system&quot; that costs $250 per year in addition to $10 per submitted manuscript (thus, if you have 200 submissions per year, it adds up to $2250).