One reason: your project is already designed around the RPi form factor, and/or you already take advantage of things like GPIO that are more specific to a Pi th

One reason: your project is already designed around the RPi form factor, and/or you already take advantage of things like GPIO that are more specific to a Pi than a "mini PC" or whatever. And it would cost enough (in actual dollars, or just in time/energy/attention/etc) to redesign the project to where it makes more sense to just upgrade to the slightly more expensive Pi.