Because the SRA was abolished and replaced effectively with...nothing? So I think policy and rules that should have continued to evolve as conditions changed became frozen in time and the TOCs became more and more confident about ignoring the parts that they didn't like and which the DfT didn't enforce but which nobody was willing to review and make some hard decisions and update. The DfT then seems to have done a smoke and mirrors "simplification" of the whole thing last year, stripping away most of the bits the TOCs were blatantly ignoring without providing any new solutions to the underlying problems. With the rewrite, the fact that it was always double the normal fare (not the highest available one) became more obvious, but this was the same in the old rules - I don't think the DfT actually changed anything in this respect last year. (The railway had just applied its own interpretation of 'full', but when you consider the whole context I think the 'without any railcard type of discount' meaning is the only consistent one.) Meanwhile some TOCs started to create alternative schemes with higher "penalties" by "misusing" the DfT's change to the byelaw 18 penalty (which has to my knowledge still not yet been examined properly by the courts, but an FOI on the matter several years ago worryingly couldn't locate the relevant files).
E.g. Perhaps the most blatantly ignored requirements were the "For this reason, we will not allow tickets to be sold on penalty fares trains unless..." conditions that were almost universally ignored and have now been deleted, which leaves the original problem they were trying to address unsolved, viz. "The basic principle of any penalty fares scheme is that passengers must buy their tickets before they get on their train. If passengers find that they can buy their ticket on the train from the conductor or guard, it undermines this message."
And I think the most serious problem for the reputation of the scheme now is the removal of the fast and separate PERTIS machines, so it is no longer possible quickly to obtain a permit to travel when the slow and complicated TVM or ticket office is busy. So the valid excuses of "the queue was too long" or "I couldn't use the machine because it was too complicated to find the ticket I needed or because I wanted to pay cash" now undermine the scheme. But new technologies are available so there could be other solutions. "Queue too long or machine not issuing tickets and unable to buy a smartphone ticket? Prove you were at the station at that time by telling me the memorable random discount code word that changes every few minutes and was given at the top of the TVM (or on the CIS or PA)." Without the correct discount code for your journey, you'll pay a higher "late purchase" fare. People these days are used to the idea of a discount code (and this code could be printed on the 'Promise to Pay' where those are available), and labelling it 'late purchase' helps with the message to buy before starting the journey.
