I think the reason Frozen Mallet, Rylais, and Ashe are popular is due to this...
The swiftness mastery from Season 5 was pushed far below on the resolve tree.
If you recall in season 4 or 5, the swiftness mastery was made a tier 1 mastery. Ashe's slow was so weak and received a rework later on. Later on 5.13 - the first mage item rework, Rylais was buffed with increased slow due to the popularity of the mastery and as a source of cc for immobile mages( between low regeneration and swiftness - pretty obvious choice). Mallet was buffed in the beginning of Season 6 and received a great build path.
Anyways, fast forward to today's mastery system and swiftness demands a decent point investment since tenacity now comes with that package. So, we have a stronk slow from Mallet and Rylais and an Ashe who loves the fact that no one worth killing can get swiftness (although this isn't exactly bad).
