Is this due to the West Coast Railway Company's ongoing battles with the Office of Rail and Road that have played out through the courts including your High Court, so some other reason?
WCRC are arguing over regulation 5 of the Railway Safety Regulations 1999 which requires central door locking.
Currently WCRC only have one rake compliant or exempt, and this is air braked only, and only one air braked main line certified steam locomotive, Tangmere. SRPS had a main line certified rake however this certification has lapsed and neither exemptions nor compliance are in place.
Other heritage stock operators (e.g. Belmond and Vintage Trains) have been granted exemption extensions to March 2028 having agreed a plan to comply with regulation 5 - hence they are still operating.
There are 2 key challenges with compliance. First is the technical: the standard BR design requires a reliable air pressure and power supply. The former is not present on vacuum brake trains and the latter not present with Steam. Whilst power can be provided by a generator, this is requires another vehicle which the operator needs to have and reduces maximum train length - which is a particular issue on certain routes such as the West Highland Line, hence the Jacobite.
Other MK1 CDL solutions (Chiltern bubble car, Hastings Diesels) require a reliable power source - see above!
The second challenge are the economics and specifically return on investment: Regulation 4 covers crash worthiness and is also being covered on exemptions, currently again to March 2028. Therefore the business case for fitting CDL has to be cognisant that there is no guarantee that the stock can operate from 1 Apr 28.
Hope this helps explain the landscape.