Because I don't understand the reason to give us ability to gain rewards only once per week, instead of allowing 2+2+1 weekly runs.
Players who are ready for difficulties will be pleased to go in this particular game mode to a bit more then 2+1+1+1 considering that it'll be much harder because of the large pull of debuff modifiers. I mostly play missions until level cap and lots of friend too, and I/m sure there are lots of players who are looking for such trials that will give fair and equal rewards.
Why not to do this thing if it's possible and easy to do? It's the same as adding in game an abilitity to upgrade multiple same relics at once only by adding 1 extra window (to place number of how much) like we buy relic packs or kuva or other products in syndicates and shop. Speending 1800 void traces after long mission to upgrade 1 type of relic( Axi V8, for example) is getting tired. And this this already if you wnna make Radiant once. Imagine you need a lot of lower grade relics (25 or 50). Funny time?