Friso94

Answer: Didn't think he had to.

Answer: While it's purely speculation, here are few ideas: 1. He didn't think he needed to. He now possesses the most powerful weapon in the universe and thought hitting Thanos in the chest would be enough. 2. He may have aimed for the head, but Thanos was trying to repel the axe, which caused the aim to be off. 3. He wanted Thanos to die slowly so that he could deliver his "I told you..." line.

Plus, he didn't know Thanos already had all the gems.

lionhead

Or just because the head is a much smaller target and Thor was pretty far away, even when throwing a magic axe.

Friso94