They take an insane amount of work, so they are slow in coming. This latest one had about 50kloc of code written to build the demo software, animations, and to improve the FOSS video editing software that was used.
Using something like OpenShot, KDenLive or PiTiVi or even Blender only makes sense to me.
But Cinelerra obviously worked out for them; the productions are absolutely spectacular. Loved every bit of this one. The clearest explanation one could hope to get.
In short, I found no FOSS video editor that did what I needed reliably out of the box. Given that problem, Cinelerra at least came close, the code was readable, and the design sound. Its biggest problem is that its file loaders/exporters are all old and bitrotting so they have become unreliable and crash-prone. For my stuff, I use raw video and so don't hit those problems.
In the process of making Episode 2, I wrote a new compositor resampler and a new color grading filter for Cinelerra (named Blue Banana). These should be appearing officially in the next major release. I hope to have some more file loader work done by then too :-)