Frozen Mallet has become an amazing item on champions such as Yasuo. It has also gained popularity on champions such as Teemo and Vayne.

The reason for the sudden popularity of Frozen Mallet dispite not always having been a very good, nor popular item is because it has a very interesting sort of rule to it.

The thing about Frozen Mallet other than ofc. the stats of 700HP (Being the highest amount of HP on an items, besides Warmog's Armor and cinderhulk ), and the 30AD being a nice bonus. But the importance of the item is the slow effect.

You see in league, the game is typically set in a way so that champions with high damage, shouldn't have high HP, and stickyness (the ability to 'stick' to an enemy champion with slows and hard CC). And champions with low damage typically do have those 'abilities'.
However Frozen Mallet changes that. It makes it so that Champions like Yasuo, Vayne, and Teemo, whom which have insane damage, also has great stickyness.

Conclusion / TL;DR
Basically the thing about Frozen Mallet, is that if a champion has higher damage than their opponent, Frozen Mallet will make it impossible to survive an engage, unless Flash, Ghost or dashes, etc. are used.