The Pink Panthers jersey started back when Sanyo was still a sponsor and was created for Panthers WIL activations.
Back then, even though it was worn maybe 1-2 times a year, it was very popular with the fan base & got plenty of attention.
I've heard plenty of stories as to why it became & returned to being the alternate kit for the Panthers, ranging from Gus made the call after speaking to Eddie Maguire, to Oak wanting it was product placement, etc.
All these maybe true to some extent, though I think the main reason are, it sells better than white (or even the goldy-brown options that were tried for a short time), it is well received by fans, players & sponsors and it is unique & recognisable.
It's definitely not going away anytime soon.