Hi,
The main reason is that most of our users are using the python API and not so many use the c++ API.
Also, the core contributor that was handling all the c++ API stuff has moved to other stuff, so there is no more core contributor to work on that.
And having a full API with high quality documentation is a lot of work…
That being said, we are very happy to take any contribution to improve the doc, API or anything related to the c++ API!