The problem with Dendrite is that we've had no choice but to focus on keeping the current servers (i.e. Synapse) scaling and alive, especially the matrix.org server. The problem is compounded by the two devs who originally led Dendrite sadly leaving during the funding uncertainty we had last year.

However, we have new folks joining to work on Dendrite over the coming weeks, and we should be able to get it moving again. But it's now looking like a two-pronged attack; one to mature Synapse to being usable - and another to keep Dendrite headed in the right direction to be a good alternative (initially for AS traffic, then full CS traffic).

https://news.ycombinator.com/item?id=16940880 has a lot more info on this.