The answer to LondonLouis is that this collection spans only Joan's A&M period. The songs he mentioned were recorded at Vanguard and it's probably copyrights matter. However, I do not know a greatest-hits-compilation that would span her entire career properly.
I haven't followed Joan's career in great detail, but I am very surprised that this album doesn't have any of the tracks from her early albums, when she was singing Dylan and was a folk super star. I would have assumed that tracks like How Many Roads and The night they drove Old Dixie down would be her genuine Best Sellers.