Why was NWS shifted from green hydraulic to yellow hydraulic system in the A320?
This enhanced package (green to yellow change) since serial number 1939[1] had the following objectives in mind:[2]

Objectives:

Avoid simultaneous loss of Normal braking and nose wheel steering functions on a single hydraulic failure. [normal braking is green]
Avoid loss of nose wheel steering function due to ADIRU or LGCIU failure. [ADR 1+3 failure results in NWS failure; ADIRU 1 and 3 control the Green shut off valve once the L/G is retracted and speed is above ~260 knots]
Make the nose wheel steering function available after a landing gear free fall extension. [L/G extension is green]


(My explanation is in square brackets.)
As a bonus, it would have permitted the eTaxi option (announced in 2013; scrapped in 2019). This way, the APU would have supplied power to the powered-wheels, as well as hydraulics for steering and braking (alternate), all without a tug, and without having to power the Green system via the PTU.[1]

1: Airbus FAST magazine #51
2: A320-family flight deck and systems briefing for pilots