DuckDB provided a lot of infrastructure for reading/writing parquet files and other common formats here. It also was inherently multi-threaded and supported being embedded in a larger program (similar to sqllite), so made it a good basis for something that could work outside of the traditional process model of Postgres.

Additionally, the postgres extension system supports most of the current project, so wouldn't say it was forced in this case, it was a design decision. :)