Because it's not based on a LLM. Apple acquired an earlier speech agent technology through a company acquisition and hasn't updated the product with the latest developments in conversational agents.
I think the OP was asking for the reasons, not the causal chain.
They clearly have not updated Siri, but why?
Because iPhone is so good overall compared to Android that people aren't going to switch because of an AI assistant that sucks.
Note that Alexa didn't get any better after it was released but that didn't stop people from shopping at Amazon.