The speech isn't illegal - the act of trading based on insider information is. This works in a similar manner to bomb-making plans being legal to disseminate, but making a bomb being very illegal.
> Why is paying someone to murder someone a different category than other forms of speech?
This is solicitation to commit a crime, which is a specific exception to the 1A.
> Why are intentionally deceptive drug advertisements a different category than other forms of speech?
This is a false statement of fact, which is a specific exception to the 1A.
> Why is pointing a gun at someone and telling them to do something a different category than other types of speech?
Pointing a gun isn't speech, it's coercion.