Silvermint, who is also a writer, created a Twitter thread in an attempt to explain why the show has veered off in a different direction by talking about the difference between writing styles of George RR Martin, the original writer of the GoT books, and the show writers.
In short, George RR Martin’s writing is style is that of a “pantser:” he tends to create rich, multi-dimensional characters with “flying by the seat of their pants” plotting. This style of writing means the creation of compelling characters, but it is prone to “meandering plots.”
Show writers on the other hand, are “ending-focused plotters by design,” due to the fact that Martin’s source material ran out, meaning they had to come up with a satisfying conclusion by planning the story from the end upwards.
Here’s the entire twitter thread, in which Silvermint explains it in more detail, below: