This happens when you have RestrainedLife enabled
and an attachment you are wearing is locked (either on you avatar or as a HUD object). Since there is no way for the viewer to know where the objects in a folder will attach when either worn ("Wear" becomes unavailable for objects) or added to outfit (note also that you cannot drag and drop a folder on your avatar while it wears a locked attachment/HUD), the viewer has no choice but to forbid these options so to avoid seeing the locked objects being "kicked off" by objects attached "en groupe".
As soon as you unlock all your attachments, it is possible again to add to outfit/wear/drag and drop (there is however a bug dealing with cached menus: if you right clicked on a folder while an attachment was locked, even after unlocking everything, the Add to outfit/Wear//etc menu items will stay greyed out for that particular folder. This problem disappears after a relog, and does not affect folders for which you did not pull down the context menu while wearing locked attachments).
For full details, please see the
RestrainedLifeReadme