It's not weird at all.
Look at Galidor, a show that only went a single year. It wasn't received well and was cut short because they didn't want to put money into something people didn't like. Nothing weird about that.
This is a ToysRUs on the American East Coast that stocks and sells out of LEGO sets constantly. There's not a lack of demand for plastic building toys here. Even Hero Factory sold through faster than this, I remember seeing only a handful of Invasion From Below sets at the end of the theme a while back while I can still see an entire section of an isle completely full of Bionicle toys.
While this is true, we also shouldn't forever be latched onto the idea that Bionicle will return. Let's face it, this community can have a darn hard time letting go of this franchise, and I think that always thinking Lego has some "greater reason" for ending it is an example of that.
There's a fine line between having wishful thinking and being desperate for something. Bionicle is not a "special treatment" LEGO theme. It lasted a while, but so have others. If it ends, it's for the same reason as any LEGO theme; there was a drop in demand or popularity. A company doesn't keep something just because the fanbase likes it, they keep it because it's profitable and makes money. That's not to say LEGO doesn't care about the fanbase, but as a company money and profit are first and foremost. If Bionicle stops making money, they have no reason at all to continue it.