Because they wanted to force the upgrade on everyone. Truth is the only thing stopping the entire map being lit-up 24/7 in season 5 was the fact that many people didn't bother upgrading their trinket or didn't even know the details of the upgrade
Last season I upgraded my trinket every game and my MMR shot up immediately. I never bought green wards ( unless I was getting camped early) because the totem upgrade was that fucking good. It was especially good for split pushing, which is impossible now.
They could have just kept the yellow trinket unchanged and promoted the trinket upgrade through recommended items and pop-up tips, that way people who care about vision aren't dragged down by those who don't, right now I feel like i'm walking around in the dark and if I try to proactively ward then my trinket is always down when I need it most.