Question

Why does Kafka devote so much more space to Gregor's thoughts than to his physical change

Answer 1

Because Gregor believes he's still a human, so it shows how he perceives the way he's being treated by his family, and how he can no longer provide for his family. 

Answer 2

In Kafka´s novella, Metamorphosis, Gregor does find himself turned into a bug, and, though important as this fact is, what becomes most striking in the novella is how his consciousness starts to change and how his family reacts. Gregor changes from being a “normal” kind of person, with no relevant features, to something new and novel: as he becomes a horrible vermin he also, surprisingly, becomes a form of consciousness that is, again, oddly enough, extremely more human and fascinating than what he was before. This is why his thoughts are much more important than his bodily condition.