Thanks to everyone for their thoughtful posts, which together present a helpful analysis of where we are and how we got here.
The first thing to say is that I personally frequently get the same feeling. I look at some of the wikitext code I write and see that soup of brackets, and think how overwhelming it must be for beginners. I’m acutely aware of the complexity inherent in flexible, open-ended tools like TiddlyWiki, and strive to improve things.
As others have noted, the basic reason is that TiddlyWiki has evolved over time with strict backwards compatibility constraints. A lot of that evolution has been directed towards improving TiddlyWiki’s wikitext and filter languages so that more of TiddlyWiki’s internal logic can be hackable. That’s why things like map and reduce were introduced, even though no end user had ever asked for them.
TiddlyWiki’s vision is that everyone has slightly different needs, and that we can only universally and respectfully cater to those needs by allowing every aspect of the experience and functionality to be changed or improved. That’s why the long term goal for TiddlyWiki is that the core becomes an ever smaller sliver of JavaScript, with more and more of the core implemented in wikitext.
We do have a reasonable plan that should let us improve things: at some point in the future we deprecate obsolete features, perhaps moving them into an optional plugin, and strip the implementation out of the core.
My feeling is that it would make sense to do that once the parameterised transclusion work is merged, and also the core templates have been updated to use the new features.