I guess some unfinished hillforts were left as such when a new-kid-on-the-block chieftain was bumped off trying to usurp the authority of a rival - sort of like preserving a town razed to the ground as a warning to others.

But I do think far too little is made of the potential symbollic aspect of ramparts.... many a hillfort's 'defences' enclose pre-existing long and round barrows and cairns which would have obviously not only provided bonus building material, but have simply been 'in the way'. That they were retained strongly suggests the inhabitants wanted to incorporate some residual spirituality of a long-term sacred hilltop into their village. To have some divine intervention on their side, so to speak.

The earthern long barrow/long cairn format had always represented spirituality (some examples having no trace of any burial, of course), so perhaps the hillfort rampart had a dual symbollic/practically defensive aspect, like the henge bank? So perhaps an 'unfinished' hillfort was the equivalent of a long barrow without a burial chamber? Hmmmmmmm.