It's easier to release music from those artist because they likely already signed away their rights or had signed things that stated what would happen if they died. Nipsey also had more ownership/partnership so the owner of the estate likely has approximately 50% control.
When you have a dispute over an estate, the disputes over who gets what percentage can swallow up any profits, especially when they're not "hit" records expected to do millions.
And some of the splits & credits may not have been finalized.
Look at the whole 2Pac sage and how the control has been all over the place and caused delays when we know there are literally hundreds of songs left. They still haven't released the original versions due to many things

. The death of so many people involved with his music makes it even harder.