I have written formal stuff for years, always writing in complete sentences, no slang, no colloquialisms, no abbreviations like isn't or haven't, whom and nor where necessary, no mixture of styles, initialize for American clients, and so forth. All clarity and unambiguous.
But my stories are full of isn't and haven't, sentences without verbs, simple words, sentences that finish with and. I break all the rules.
Why the big change?
It is a question of style.