IBG has nerfs on the PBE.
Frozen Heart was, I believe it was 200g, cheaper last season and had 10 more armor (It was even 150g cheaper than that beforehand). Last season though, just that minor difference made it a rush item on almost every tank. It was insanely core. Every top laner, support, and tanky jungler built frozen heart.
It's still a good item, just outclassed by IBG. Why go for a small AS slow and slightly more armor when you can get the extra damage and a slow-on-hit instead (for less!)?
Frozen heart will likely see more play once IBG + Sunfire cape combo gets cut down a bit. The combination of armor + 20% CDR is really good, and the 15% AOE AS slow is really valuable too.