The holotype needs to be accessible so that other paleontologists have the opportunity to determine if something that they have found is the same or different species.
So if all we find of a ceratopsian skeleton is the rear portion of it, we would not be able to confidentially say if it could be the holotype for a new species.
New fossils are found all the time, and paleontologists need to be able to determine if what they've just found in the field is a brand new holotype or another example of an already described species.