And I'm sure they would've done them all, but for the fact that their hearts weren't into it anymore. As Oxbeard rightly said, Weta is an effects company, and their collectible output is more than dictated by the movies they're working on at the moment.
Then again, there is Weta Digital, which doesn't exactly seem overbusy after the completion of Kong, and there is Weta Workshop, which continuously works on a multitude of projects. Yet they're able to do collectibles on top of everything else. I think they did get a bit cocksure after the LOTR line took off and thought they could go at it alone, but in the end they might be sorry they didn't share some of the risk of this particular venture. And once the dust settles over Kong, and we get a clearer picture of the viability of the Narnia line, the artists at Weta will probably make a move toward reconsidering the LOTR collectible line on their own. Sideshow by then might very well dominate the LOTR market with their own lines, thereby eating into Weta's potential.
Of course, a Hobbit movie might change all that, but that's still a long way away, and is actually a separate issue.
Well, it's too early to speculate, anyhow. By the end of this year the picture will be much clearer, hopefully to the advantage of us collectors...