What motivated Spielberg to direct this exactly?

I was surprised how disappointing (do I dare say, awful) this movie was. And this is coming from a huge Spielberg admirer who ranked The Post as one of the best films of that year.

There's really no story, scenes seem to drift languidly with no purpose, and I remember the book having immense fun with language and BFG-isms, and the friendship between the girl and giant was very sweet. Even if I put on my "kid hat" and look at it through their eyes, I cannot see them enjoying this much. It's actually too dull and lacks the wholesome whimsy that much better kid films have.

But to answer your question, I suspect this movie was a quick exercise for Spielberg. It did not feel like his heart was in it, and it felt more like a rushed effort to tinker with "Virtual Reality Mocap" filmmaking techniques (seen in Avatar), and used in Tintin and Ready Player One.

(edit) To be fair to the movie, I initially did not finish it. I was that bored. I just went back to finish it and the scenes with the Queen actually made me perk back up. The charm and whimsy I wanted was sort of coming back in these scenes. Shame the majority of the movie wasn't like this.