Demand and supply?
The red's popular as it makes a dish look more interesting and so, appetising. I thought that reds were maturer green ones and that possibly, yellows were the maturer of the three. Do they grade in flavour too, that is piquancy?
Seriously impressed by Walk2's sense of priorities: "...(so) I can spend my money on important things, like walking trips."

If you don't walk, you don't eat and if you don't eat, you die.
Remember that Gaganis' in Hindmarsh sell imported dried onion, quite reasonably priced. Lots of good stuff there.
Surgite et .. andiamo!