Because it is not necessary. The actual parties involved know what is meant and intended, so adding some ongoing and constant amendment process is counter-productive.
Putting on my govt contractor hat, there may be a business opportunity here to set up VMs running Win95/Netscape Communicator for use by all the civil servants looking to comply with the law. Could charge a pretty penny too - it’ll all get budgeted as “Brexit compliance” costs.
The text quoted in the linked article in no way mandates the use of Netscape Navigator or Mozilla Mail - it merely references them as being widely distributed software capable of using RSA 1024 and SHA-1 (which it does appear to mandate).
That reminds me of FIPS for some reason...
Sometimes they don't exist - who maintains a sufficiently reputable list of safe email clients and web browsers?
Sometimes they exist but they carry some baggage - the FIPS standards for cryptography are probably fine, certainly better than hardcoding a couple of algorithm names, but they're also controlled by a foreign government.
Trying to keep lawyers in business it sounds like. “Intent” is one of the trickiest things in the world when you want it to be.