Why? A lot of the missions have criteria that can be done in ranked, and that should always be done in any match.
Last weeks: Earn 100k gold. Why the flying fuck should that not be enabled in ranked? It's something that you literally cannot avoid doing if you play. Everyone earns gold in every match. You could make a case for the herald/baron one, since it could detract from a more optimal objective, but it'd be a weak case.
This week: Kill wards. Well, you're supposed to kill wards, so no issue here. And if people are fighting over killing a ward, guess what? It has a maximum of 4 hitpoints. It doesn't matter how many people are fighting over it, it's going to die quickly. Nobody's going to be able to kill someone else that quickly.
Win with IE and win with Randuin's. Well. There's only one marksman that usually doesn't use IE, and that's Ezreal (and he probably does, anyways). A decent number of assassins and skirmishers also pick it up occasionally, since it has a lot of AD. Any marksman that doesn't buy IE is an idiot. Nobody's going to rush it over anything else, unless they would have anyways.
You could make a case for Randuin's, as not every team needs it. But it's not a useless item, even against high AP comps. The active slow is powerful, and useful against pretty much everyone. And you're not going to see a team without a crit user, anyways.
Your argument is completely pointless. Why should I have to complete a simple mission that I'm going to do in any ranked match I want to win in a normal match? Hell, even the troll-like mission of the melee ones was premade exclusive, so that you couldn't screw over someone else no matter how you tried to do it.