I’ll throw in some thoughts.
This is a niche within a niche product. That doesn’t mean it can’t grow more. But, the OP asks why someone would visit then leave. In a nutshell that’s my opinion why… Niche within niche. Which I love. That’s why I’m here. But, I feel like many people settle for a basic notes app, blog, or even pen and paper.
You also must be a little tech savvy to use TiddlyWiki. Not a lot. But, a little. You may think dropping a HTML file somewhere is easy, but even easier is clicking download in an App Store for something else. And there’s no saving intricacies with a free downloaded app.
Those are reasons why “regular users” may not use it. But, maybe you’re wondering how to attract developers? I have no help for you there. I don’t know what developers look for when deciding to contribute to these projects.
To overcome the above “regular users” problems, as others have said, a clearer landing page could help. Maybe both visually and textually hammering home that this amazing tool is one file. And showing that you can just drop it on your desktop. And, that your notes are forever your notes. Unlike if an online app gets hacked or goes out of business. Then after some eye catching things like that, go into the explanation such as what is currently there at tiddlywiki.com.
Personally, the number one reason I come back to TiddlyWiki time and time again is because I feel like a single file is more likely to store my thoughts longer than some other app. Again, there’s no company going out of business or risk of getting hacked (well, less of a chance especially if kept offline). And in a worse case scenario, if I have to sell all of my processions, hopefully I can at least keep a USB drive with my TiddlyWiki on it.