Over time, maintenance grows
When starting an Open Source project there is a lot of code to write and to polish. Maybe even to rewrite. Lots of features to add.
Over time, software projects tend to mature and the speed and frequency at which you add features slow down and more time is spent on fixing bugs. Architectural questions become harder and you get more technical debt that locks you into sticking to certain ways. Or at least they make new features require a larger effort than they did early on in the project.
I do most of my work in the curl project answering email, trying to reproduce and understand people's bug reports, clarifying documentation or blogging about a related subject. Only a small fraction of my curl time is actual development time.