Why is my pullup resistor more noise immune than a pull down?
PCF8574 I/O already has pull-ups in it, in the form of a 'weak 100 µA current source'.

If you try to pull the input down with 10 kΩ then the 100 μA internal pullup current flowing through your pull-down resistor will raise the open-circuit input voltage by 1 volt, reducing your noise margin.
With a 10k pull-up you are improving the noise margin because your resistor and the internal pull-up are working together instead of fighting each other.