Hello,
Was just browsing and saw your post.
I am both a game jam host and game jam submitter, so hopefully I can try provide some input. Whether this will help you or not I'm not sure, since I am pretty much a fledgling in terms of experience in both fields.
To drop some metrics, my first game jam had around 55 participants with 15 submissions and around 50 ratings. This honestly exceeded my expectations; I had no outlet to promote the jam other than it's itch jam page. Considering this a success, I quickly announced my follow up jam and even expanded the prize pool expecting the community to grow. Result was 18 participants with 2 submissions (the jam is still in voting period).
I have drawn two theories as to why this was. Firstly, I believe I made my second jam far too niche and therefore put potential participants off. Secondly, simply bad timing on my part. The jam coincided with the favoured official annual jam in the game engine of my choice and thus participants likely opted to join that jam to rightly give their games more exposure.
I took a quick look at your jam page. It's obvious you care about what you do which is admirable, but I can see a couple of potential turnoffs (with my jammer hat on not my host hat). I think it seems a bit too restrictive in what the devs can actually do/make. Having restrictions/challenges is fine and fun but you don't want to make my mistake and put people in too much of a straight jacket.
Secondly, a big turn off for me personally is having a theme announced just right before the jam starts means people cannot even brainstorm their ideas ahead of time. Sometimes I like to have an idea and then find a jam to fit what I've got in mind to give me the motivation to get it done. Having it the other way round sometimes stymies the natural creative process, I find.
But that you've reached out means you care enough to want to make it a success and that's what's important. I wish you the best!