Dave:
That, again, is the whole point, Nigel. Why should the treasure belong to the crown? A remnant of feudalism.

I believe I answered that by implication. If the ownership of 'treasure' was not vested in the crown then it must lie elsewhere. That would be with one of: Original Owners, Finders, Landowner (there could be other possibilities).
It then means that there is nothing to stop the finder from taking and disposing of the treasure as they wish. This would lead to great losses for landowners and the archaeological purists, and our knowledge of British history.
Having a central ownership of found treasure allows the government/crown to insist on rules on how finds must be handled. The crown's ownership, as I showed, does not mean that the crown owns the value of the treasure (this is available to the finder/landowner) but can restrict how the treasure is distributed/disposed of.

If it was not vested in the crown we would have to find another interim owner to provide the same safeguards.