Classic example of to each his own, how vastly preferences vary between users.
For my personal preferences, I find that while a journalling app could/ may support markdown and that would be a huge plus (as the output would be formatted to my preferences, if and when I want to output for others to read) but for the specific purpose of journalling, the less the clutter, the better the journalling.
Journalling can be seen in multiple perspectives. Some see it as a scrap book, some see it as detailed account of their day, replete with photos and what not, some see it as a therapeutic medium, wherein they are vocalising their thoughts for themselves, and for their therapists and such. This last form of journalling use is what I feel was the original target experience of Dyrii, a simple and pure piece of digital canvas for you to start penning your thoughts on.
The provision of templates, and the featured template (Making SPACE) is testimony to it. The other templates also give an idea of who for and what for is Dyrii generally intended.
To such an audience, not having markdown would not matter at all. All they would want is a clutter-free, no nonsense, no frills experience where nothing comes between them and their thoughts, not even the canvas. And this is where Dyrii excels at. You make the app full screen, turn on dark mode, and boy, the beauty of that calm black screen with subdued colours makes for a very effective environment to just release your thoughts on the digital canvas. Having to use markdown would, hypothetically, as well as literally, slow such people down.
Markdown would be a distraction in such a case, because people are not wired to think in markdown, they are wired to think in English. While typing, using symbols to effect markdown would cause a slowdown, therefore.
