I only have two explanations. Blocked members and banned topics.

It's constantly going downhill with the discussions. Yeah, due to the April purge many active forum users were forced to leave but there was a downward trend already. Three years ago, this was a whole different forum with lively and interesting discussions. And well, users who've been here longer will tell you that five, six or ten years ago, it was a whole different story. I miss all the users who started interesting and intelligent linguistic discussions. So many good people were either blocked or simply lost interest in the discussion forum.  

Alex, for what I could read in a few threads, it seems that last year there was a big purge on Italki. I ignore exactly when and why that happened. What I know is that, after being excluded,  some ex-italkiers organized their own forum where they could, and still can, discuss freely and without any fear of a possible ban. Apparently, other active italki-members migrated to the forum. As a consequence, the discussion section on italki has become quite empty and bland since with respect to the past. 
This is the link to the forum:  evengreener.createaforum.com/general-discussion/

Charlie Collingwood, if you give a look at the discussion lists in the forum above, you'll have a hint of the topics that they probably tried to discuss here, and that caused their exclusion. 

But again, it's just a guess. I was not around at the time. 

Alex, you said: " Religious and political discussions would deter potential customers."

Actually, I rather think that free political discussions as well as religious criticism or proselytism would make this site immediately banned in all those countries where such activities are prohibited or strongly limited. Which would bring an economic loss for both the site and the people who sell their services on it.

Here, a list of the countries that most heavily censor subversive materials online: 
www.le-vpn.com/top-10-censors-internet-avoid-internet-censorship/
With the exception of North Korea, I think that italki is granted access to all of them.

In order for an Internet page to be displayed in the whole world, included in those States with limited freedom of speech, its contents must be strictly monitored. It's even the necessary condition for its (virtual) survival.

Mind you, I'm not defending the model. I'm just voicing what is probably one of the italki-management's main concerns.

(edited for grammar mistakes/typos)

I'm going to offer a differing opinion ... I find it annoying when people try to inject politics into everything, like sports for example.  Isn't there somewhere we can go to escape from 24x7 political propaganda?   It just seems to make people angry at each other, and no one is converted by these discussions.

There's plenty of other things to discuss besides politics and religion.  If you want to argue those things then I'm sure there are better forums out there for that.