I wonder what the rationale is there - I had been pondering that it
was some sort of systems issue (that I couldn't quite decipher), but
actually I wonder if it might not be more of a safeguard to ensure
that those who have access to an Oyster card's journey history really
are the bona fide holder of said card. Something to scratch my head
over...

I always thought it something to do with the online Oyster ticketing being
operated by a third party, e.g. Keith Prowse or somebody like that, and
their only being able offer the journey history to their own customers. No
idea how I formed that impression though and I imagine it will turn out to
be a load of drivel.