Cost saying is some reason I considered too.
If I think back to the fight over cheap steel from China which there was a fuzz about last year...

Other than that, I can't come up for a reason why one should start to make those parts of plastic.

They aren't that weight-heavy while pushing around, but seriously, put two bags of potatoes in it or a six-pack of any sort of drinks of your choice and then this thing gets harder to push around again... So, what the heck...?