Probably because BIONICLE wasn't exactly built for this generation.
BIONICLE as far to my knowledge was one of the first products or toys to use online marketing, games, etc. It was brand new and was fascinating. It went on for a long time.
Fast forward to 2015 where, by now, strong online presence was a common tactic used by LEGO so the new BIONICLE wasn't groundbreaking. Not to mention, constraction was a pretty niche area in LEGO building. In addition, there were already a lot of LEGO themes that kids stuck to already that sold way better and seemed more engaging than BIONICLE. Such examples include Marvel and DC, Ninjago, Nexo Knights, and Minecraft. They outsold and appeared more appealing than some action figures.
BIONICLE needed to come up with brand new formulas to become a decent successful theme. It did tread some water by going into Netflix, though I don't think LEGO emphasized that very well. New games from BIONICLE weren't shown as much as other LEGO games; for example, Nexo Knights, which had a game that encouraged buying sets. That's what BIONICLE should have aimed for, in my opinion.
TL;DR - BIONICLE was well built back then when the internet was new, however what it helped pioneer has become mainstream and to a normal consumer, BIONICLE isn't engaging enough to warrant buying the sets. Media presence of BIONICLE doesn't encourage buying the sets. More popular and less niche lines like Ninjago or Marvel exist.