Well, in my view, it's because GBR and nationalisation of the franchised TOCs is nothing but a vote grabber. The creation of GBR, as well as the nationalisation of the franchised TOCs, doesn't necessarily mean anything will actually change other than who runs the services.
If the government was serious about nationalisation for example, they would nationalise the ROSCOs.
If the government was serious about fare reform, they could do that without state ownership of the TOCs already.
In fact, it's just that both political parties have clocked the fact that the general public say in surveys and opinion polls that they want the railway renationalised, and this appears to voters to do that.
While I'm sure the DfT will come out with some sort of fares reform plan, I wouldn't get hopes up about anything revolutionary.
Additionally, GBR doesn't cover all rail services. Lumo, Hull Trains, Grand Central, TfW, ScotRail, Merseyrail, Elizabeth Line, the Underground, and I suppose the Eurostar for what that's worth in the context, won't be covered. So the fares system we currently have is still going to exist regardless of GBR, because a lot of these operators have route or operator specific fares.