Frozen Mallet is anti-fun and hard to balance, but Entropy was never brought to Summoners Rift?

There was a time in which Frozen Mallet needed to be toned down for its high amount of dominance, since it could destroy a lane even if you were 0/2. Riot had taken Phage out of the build path, and tweaked the values of Frozen Mallet. Few people actually use Frozen Mallet anymore, because it is very expensive and opens a lot of opportunities for enemies to fight you, even if your under turret. There is an item called Entropy, it cant perma slow people, but it has a pretty powerful active: UNIQUE ACTIVE: For the next 5 seconds, basic attacks reduce the targets movement speed by 30% and deal 80 true damage over 2.5 seconds on-hit. This is a very cool item compared to Frozen Mallet, the stats are cooler, the passive and active are cooler, even the icon for it is cooler. What I don't understand however is why you don't just replace Frozen Mallet with this item? You could easily keep marksman from ever using it by going (Melee only) and if it actually had a uber strong active, the cooldown or gold value could be raised. I don't understand why this item isn't brought to Summoners Rift, it would really help out the meta where Last Whisper and Void Staff force tanks to get on they're knees. Before anyone says that the item wouldn't even be used, I can give three champion examples that would benefit from it: as a jungler, he would be spectacular with the attack damage, health, ect. He is in a decent spot right now but he doesn't have a real item to transition him between a tank and fighter, like how a Vi or Sion would build Trinity Force because it gave burst and sustain. would be stronger, he would be pretty strong with this item, maybe too strong. Why is it strong? His E benefits from the attack damage, and his passive scales well with the health. He would also gain alot of chasing power from the active and passive. Renekton is would be good with this item too, he could build Trinity Force but he doesn't really go well with it unless he builds cooldown reduction, not to mention the useless mana. Obviously the ad scaling is strong on him, and the health is good too. And with that, I still have the question: Why hasn't Entropy replaced Frozen Mallet on the rift yet? Frozen Mallet lacks counter play where Entropy does have counter play. Entropy is by default more healthy compared to Frozen Mallet because of the perma slow. The player building FM is going to be a walking Poro for the enemy team to kill before they get the item, and that's all the counter play to it. Entropy doesn't perma slow, the price could easily just be raised to balance out the power in it, and it should be brought to the rift