Because GRASS doesn't play nice others. Calling GRASS from other apps isn't as easy as one might like and GRASS has a very GRASS-specific way it want to do things. Which is a shame because most of GRASS's algorithms are really best in class.

I agree with you. That being said, it's a go-to tool for me when I'm cleaning road networks and for the subsequent network analysis. And that's a hugely underwhelming use-case, I'm touching a fraction of what GRASS does.

Very helpful community as well.

If someone made a friendly python GRASS wrapper that would let you use GRASS function in a pythonic manner with numpy/rasterio/fiona/shapely data i think GRASS usage would increase massively.

Yes, this is it for me too. Whenever I have a huge vector dataset I need to clean up, I would use GRASS simply because it is topological, and can re-interpret, fix, manipulate vectors efficiently and valid, as a single layer entity. Even though PostGIS added topology, I have not been able to figure out how to do the same manipulations, or achieve the same speed.

A GUI from the 90s, documentation often indecipherable if you are not a subject expert, awkward UX.

I like using it via QGIS but that's as near I can make myself go near it. It saddens me because it seems very powerful and featureful.