Two reasons - (i) tailwind is not really a good start if you want a nice set of design patterns already built for you (sure there are good tailwind component libraries, but HARC is an antidote to JavaScript I hope) and (ii) HARC leans into FP (Air::Functional) for HTML code and a semantic HTML library such as Pico helps to avoid clutter in the source code and imo improves maintainability. But, unlike TW you need to submit to some kind of structure for your style … but HARC components help to maintain a good locality of behaviour (LOB).

Nevertheless HARC will add some TW themes soon - next on the list after Bulma.