Why the Gap?

Disabled people were not a priority and were kept in workhouses and
asylums when rail systems were built.
The UK built some of the earliest rail systems, but they were never
intended to be joined together and so there were no coherent
standards.
To this day different rail companies have monopolies of different areas
and each have their own approach.
There is still no enforcement for platform placement standards or of
train floor heights.
New trains and platforms are still being built and bought that are not
compatible (e.g. Thameslink, Crossrail, Elisabeth Line, HS2 etc.