I went to many meetings with GRO at the time the on-line index and the pdf downloads were introduced (in 2016), so the lack of marriage digitisation was a topic that came up a lot then and they are well aware that it needs to be done.
As far as I remember, the issue is just really one of money - the provision of historical records is only a very small part of their role and most of their resources are used for running the current systems for registration, so the historic side gets a very small proportion of their funding.
This is compounded by not being allowed (by law, as a government dept.) to set charges at a level that would make a profit - they are only supposed to cover the costs and so they can't generate the money needed themselves.
The subject came up again during the trial phases of the digital download system that has just gone live, and now they are currently running some "focus group" type exercises with GRO users looking at developing their future services ( I was interviewed only a few weeks ago), so they still keep getting reminded.
Fingers crossed it will happen.
