For the past, let's say 5 years sales for Constraction have been on a definite decline. This has caused Lego to cancel various themes, such as Bionicle...
Bionicle was fairly popular (we wouldn't be on this message boards without it), so why didn't that save Constraction the second time? The sets were decent, and the story wasn't awful. What has caused such a decrease in the overall interest in Constraction?
Well, let's take a step back. In the early to late 2000's, action figures were extremely popular. It was only in the 90's that balljoints popularized for a brand new way of making action figures posable and playable. Naturally, this contributed to the boom. Transformers toys were more posable than ever, and new lines also began to develop, such as Slizers and, a bit later, Bionicle.
What does this have to do with Bionicle? Quite a bit. This was an era where a gaming computer and a smartphone didn't exist in every house; the era where imagination and television were basically the two main forms of entertainment for kids (provided you didn't like sports ), which required you to rely a little less on something tangible. With this, "imaginative play" was popular, you know, pew pew bang bang. Not relying on something visible to have fun, because that's just the kind of entertainment you had back then. Video games were still primitive, and not extremely cheap. You had to be a little imaginative and abstract when playing with toys.
Now, back to Bionicle for a second. What could you do with Bionicle back in the day? You could make something abstract. Even if your Rahi made from a scrapped Onua Nuva didn't look how you envisioned it, you still saw it as that, as it was abstract enough for you to tell what it was (same thing went for video games, as the earlier games required a little bit of imagination with some of the designs). For this, Constraction was very popular. You were able to make things of any shape because you knew what it was, not because someone else could.
Why'd I bring up video games? Snap back to about 4 years ago, when a game exploded. Minecraft. This game was everywhere, it was a smash hit with the younger audience. It was a sandbox. You could see what you made, and you had unlimited resources to do so. Things like this beat out toys, because why play with toys when you can do it virtually without spending any more money than the first time? Imagination became slightly less required, and you could rely more on visuals than imagination to see what you were doing.
Tying this all back to Constraction, action figures are slowly dying. I, for one, love action figures. But what about video games, especially sandboxes like Minecraft so much more popular? It's the structures. Action figures are large, and they always have been. It's expensive for a company to make a big tank for G.I. Joe's, or a Star Destroyer playset for your old Star Wars Imperials. Now, this is where the popularity of Lego System comes in. What makes System so great? You can make massive structures for your Minifigures at relatively cheap prices. Just today I bought Merlock's Library 2.0, and it was only $25 at retail. That was a decent structure with a small vehicle and three figures. Prices like these just aren't feasible with real action figures. It's just nice to be able to make some big, precisely detailed thing rather than let imagination take hold when you're making an abstract creature from a cannibalized Tahu Master.
So, what am I trying to say? Well, I rambled a bit, but it's basically this: (TL;DR)
Constraction, especially just CCBS, is too abstract to compete with something like System that offers such a more detailed output. It's so much easier to make a big structure to play in with System than it is Constraction, and you don't need to rely on imagination because everything looks the way you need it to.
I love Constraction, I always will, but I think that when video games offered a more visual source of entertainment, the tangibility of System just blew away the old-styled action figures Constraction offered. Video games already threaten the toy market, and System just allows a better competitor for that appeal than Constraction can.