It's one of the things that's fallen by the wayside with the transition to a new generation in NBA 2K14. A few of the roster editing functions were lost at that time, though some returned beginning with NBA 2K15. Hopefully 2K will be able to expand on the customisation functionality with NBA 2K18. We have seen other features make their return in the last couple of releases, including the "Start Today" option for MyLEAGUE in NBA 2K17.
As for why certain features and functions go missing, I talked a little about that - both the technical and design reasons - in a
recent column.