Long ago, when I worked at MongoDB I created Flask-PyMongo to make it easy for programmers using Flask to use the database. Fast forward almost 8 years, during which time I wasn't a consistent user of either Flask or MongoDB, and Flask-PyMongo has fallen into disrepair.
MongoDB, PyMongo, and Flask have moved on, and Flask-PyMongo hasn't been kept up to date. There are more than twice as many forks as pull requests, a GitHub ratio I'm not proud of. Fotunately, the future for Flask-PyMongo is bright.