Here's why:
* He was poisoned: William died under mysterious circumstances, likely poisoned by his own brother, Claudius. There wasn't time for a will or inheritance distribution.
* Claudius took over: Claudius, being William's brother, immediately took over the throne, and the kingdom was left under his rule.
* Hamlet's inheritance: Hamlet's primary inheritance was the right to the throne, which Claudius stole from him. He also received the title of Prince of Denmark, but that was an honorary title, not a physical inheritance.
Therefore, Hamlet's grief and anger were not just for his father's death, but also for the loss of his rightful place as king and the betrayal of his uncle.