No, I am saying that there is nothing special about passenger demand or flows in many parts of the Network Railcard area to justify different arrangements.
But it is hard to say that there is any logic between the target group for a given railcard and the specific restrictions applicable to it. They seem to be unco-ordinated accidents of history.
Ok let's have a go at a few different restrictions and why:
Disabled Persons Railcard
Discounted travel available at all times. Quite right in my opinion.
16-25 Railcard:
Discount available at all times subject to a minimum discounted fare of £12 on weekdays (doesn't apply in July and August). This allows younger workers the opportunity to get a discount on longer distance commuting (e.g. into London and other cities). Younger workers at the start of their careers are less likely to be earning a high salary, and a 34% discount is a larger discount than many season tickets gives and is also an opportunity for these railcard holders to purchase tickets on a daily basis thus spreading the cost.
Senior Railcard:
Discount available at all times except for journeys wholly in the Network Railcard Area where discounted tickets cannot be used until the Off Peak Day Return becomes valid. A but if a tricky one but there is a need to prevent these railcard holders using discounted tickets to travel ito London for commuting purposes as it would abstract too much revenue. Less of a problem in the rest of the country due to smaller commuter flows and lower ticket prices.
Network Railcard:
Target market is leisure traffic in London & South East. Objective is to promote the use of trains outside of peak commuting times into London, where fares are generally higher than in other areas of the country, hence valid from 10am on weekdays. Discount available for more than one person to encourage group travel to fill off peak trains. The bargains start when the rush stops as the adverts used to say! Minimum discounted fare was introduced to placarte some train companies who threatened to pull out.
Do I think there should be a national version of the Network Railcard - yes! It's unlikely that it will happen as there isn't generally the spare capacity to fill in the same way there is in the south east, and the undiscounted fares are generally better value outside the south east.
Family & Friends Railcard:
Offers a decent discount when travelling with children thereby making travel more affordable. Restrictions in the morning peak period in the Network Railcard Area to protect morning peak capacity for commuters. Historically less of a problem in other areas of the country.
If the terms were harmonised I'll leave you to guess what would happen, but it would result in price increases for many. Simplification sounds desirable but it really wouldn't be desirable. Be careful what you wish for!