As to why it had to be set before TOS...welp, to be honest, why not? I don't think they've done a good job showing us why this story had to be told in that time period... but, I do think someone *could* theoretically do a good job of that and I wouldn't want that person restricted by ancient aesthetics, either.
I'm wondering if the story they told was one that would of been better served had it been set beyond Voyager, beyond known canon and continuity. One of the biggest criticisms leveled at Discovery is that it looks too far removed from what we know to be that period of time.
I agree with you that the TOS cardboard 60's look isn't going to cut it in 2018 but that is the established look for that period of canon/continuity so why deliberately go back there, change the look of everything, tell a ham-fisted, hamstrung story that is ultimately going to piss more fans off than generate new fans.
Everything I've said about Discovery being great I truly believe, but there is another side to that coin and it's a bit of a mess to be fair when there was so much potential in either staying true to the aesthetics of TOS (I don't mean cardboard sets, I mean staying true to the colour palette and design style) and telling a very modern, contemporary tale within the time period or indeed setting Discovery 50/100 years beyond Voyager and creating their own vision of the future from our world as it is in 2018...
Having thoroughly enjoyed Discovery despite it's failings IMO, They've redesigned things and for what reason?