I’m guessing it’s this.
Shit’s a great, all-purpose, word. You can add as many syllables as you need to cover almost any situation. If you’re feeling relaxed, you can elide the t sound. OTOH, you can always hit the t sound hard if you’re feeling irate.
All in all, I’m not inclined to take much nonsense from a Limey while the Shyte Brook still graces your landscape.
Now fuck, on the other hand … Fuck is a much funnier word than shit, of course. Fuck has the advantage of being a K word, and rhyming with Duck, so naturally, it’s the word or choice when feeling fiotous. Also, fuck works better as a gerund, so it’s situationally more entertaining.