There are actually a number of reasons NOT to contribute to Bowtie (mainly due to it being an educational language designed to be used in a book):
While the code is open source, I'm compensated for it indirectly via sales of the book. I don't expect anyone to work to help make me money, and so am fine doing all the coding myself.
Because it's meant to support a book, I need a deep understanding of all parts of the code. To understand things I usually need to do them myself, so I'm not likely to accept PRs for big features or changes.
The codebase is meant to be understandable by new Haskellers. One of the tools to do that is a distinct style (e.g. note the preference for fmap over <$> and parentheses over $) and consistency of style is more important to it than most other projects.
Opening issues or forking Bowtie are likely to be more fruitful than making PRs (forks are especially satisfying, as Bowtie is meant to be a reasonable starting point for language experiments).
If want to make a PR anyway, probably ask first, and then in the PR add your name to CONTRIBUTORS.md.