It’s because of her professional background, you see. From Astead Herndon’s New York Times profile:

When Harris speaks in an interview or to an audience, it can sound as if she’s editing in real time, searching for the right calibration of talking points rather than displaying confidence in her message. It has contributed to a reputation as a politician who delivers “word salads,” but [Jamal] Simmons — her former communications director — argued that it’s a consequence of her career as a prosecutor and attorney general, law-enforcement roles that did not ask Harris to communicate with the press and the public in the same way.

Right. Prosecutors and attorneys general don’t do a lot of talking to the press and the public; that’s why they so rarely run for other offices and win.