Urm why should people be punished and pay a premium to travel to work? Just because you are commuting this should not equal a higher fare. Many people commute off peak, should they be taxed in this way too? It is not an incentive at all.
There aren't many jobs which allow you to start after 09:00 anyway. In fact turning up to work late or requesting a later start time is very common because of strict peak hours. Many use the fantastic excuse that price of the train ticket is cheaper so that they can enjoy a longer lie in bed in the morning. Younger workers on low wages especially are price sensitive and the railway would gain more of their custom if they were less difficult
As Railcards are a commercial product, allowing their use at peak times woild only be justified if there were a net profit for the operators overall.