"This feature is a bit out of scope"(if applicable). "Our team, aka me, has limited time for this project, and I just don't have the bandwidth. I can answer questions or possibly collaborate if you'd like to have a go at the code yourself though." "This is a personal/internal use project made available to the community as is, and adding features outside of what I need for my own company is not a priority. You're welc…
There's another consequence to consider as well: even if a PR does materialize externally, do you want to sign up to maintain it in your codebase?
In my experience pull requests, particularly for (larger) features, can be net negative contributions. If the pull request is done poorly they can easily take more time to review, fix and maintain than it would have taken the maintainer to implement the feature in the first place.