The variation of answers kind of proves that none of them actually are fully informed about it, only one of these actually seems aware the sets are returning. The rest are likely just toy store members who don't really have a clue about how the industry works.
To clarify, it was not directly due to sales being poor as this insinuates. It was just a lack of new people buying the sets. They were achieving the goal they wanted, but they still wanted more than they were getting.
For example; They wanted to sell 1000 sets but they sold 1001. They still made a profit and sold the sets they wanted but they didn't sell a lot over the amount they wanted to sell.
I keep seeing this complaint, however Bionicle's storyline was incredibly simply 'good guy toa vs. bad guy makuta' it never really fluctuated from that. The storyline itself was never complex unless you chose to delve into the comics and stories. You could easily buy the line or follow the story without delving into it too much.
Granted the main story spanned 10 years, but every year it was essentially the same storyline over and over. Eight years of which could be summarised in less than 12 minutes;
Bionicle always made a profit, the issue was that it wasn't making enough. Stores didn't stop selling the sets because of a lack of profit.