Learn Russian: Почему ты всегда вешаешь сумку на этот крючок? - Why do you always hang the bag on this hook?

Вешать (you hang: вешаешь) is a transitive action, meaning you are physically putting an object somewhere. It takes a direct object (сумку). Висеть is a state meaning "to be hanging". If you used a form of висеть here, you would be asking why the person themselves is dangling from the hook!

Why is it на этот крючок (Accusative) rather than на этом крючке (Prepositional)?

We use на + Accusative because вешать is a verb of positioning. You are directing the bag onto the hook (the destination). If you were describing where the bag already is using a verb of state, you would use the Prepositional case: Сумка висит на этом крючке (The bag is hanging on this hook).

Is вешаешь perfective or imperfective, and why?

It is imperfective. The word всегда (always) tells us this is a repeated, habitual action, which requires the imperfective aspect. If you wanted to ask "Why did you hang it there?" about a single completed action, you would use the perfective verb: повесил.

If на этот крючок is in the Accusative case, why hasn't the ending changed?

Крючок (hook) is a masculine, inanimate noun. For masculine inanimate nouns and their modifiers like этот, the Accusative case looks exactly the same as the Nominative dictionary form.