Math support wasn't his personal priority. From the FAQ: "Does it support mathematics? Not yet. I understand that mathematics is important in scientific type

Math support wasn't his personal priority. From the FAQ: "Does it support mathematics? Not yet. I understand that mathematics is important in scientific typesetting, but SILE is targeting a particular niche in the humanities. That said, it's not impossible, and it should not be difficult for someone to port MathJax from Javascript to Lua and implement mathematical typesetting as an add-on package." I don't see what is particularly puzzling about implementing the things that matter to you personally, and leaving other things to contributions. I especially don't see a problem with leaving things to add-on packages if it's possible to leave them out of the core. Papers with complex math in them are a pretty small percentage of the overall number of books/articles in the world. Also I like the aesthetic. YMMV.