just speculation.
1 - reduction of rewards for helping other people clear. This is a handicap to prevent overgearing others too quickly. Also, a majority of people should not be attempting the instance again for the week after clearing. You're "meant" to be doing other things - playing other games, engaging in other in-game activities like crafting/gathering/big fishing, farming other EX content, doing older savage for gear/glam/mounts, etc. I personally myself do savage but I have no interest in doing any fight more than once a clear per week. (at least until unlock).

2 - put plainly, progression lock is an artificial subscription time extender. Can't let people get their stuff too quickly, bad for business.