Hi Venkat,
In short, delta required some kind of information in order to work (e.g. timestamp/change log), and the objects you mentioned do not have any of them.
Say for example, a standard DSO can use as a delta source as system maintain change log information while update, but in the case of "DSO for Direct Update" as change log is removed to speed up update process, so it is not possible to use as a delta source.
And it is more or less the same for InfoObjects and InfoSets, the only difference is the required information did not put into the objects at the very beginning. (i.e. it is by design)
Regards
Bill