Why isn't Curse or Son included in The Pink Panther DVD boxsets

I bought The Pink Panther film box set the other day, but I can understand why almost every Pink Panther film boxset doesn't include either Curse or Son of The Pink Panther. I mean Return I can understand, because it's owned by a different distributor and Inspector Clouseau many don't class as a Pink Panther film. But why do almost all the DVD boxsets stop after Trail of The Pink Panther. Surely if MGM do a film series boxset on DVD, they should include all films of that series. It would be like The James Bond film series missing out Licence to Kill from the boxset because it underperformed at the box office.