In a lot of instances, the change is made complicated by the lack of packaging products in newly sized pacakages with even (or rather, "whole") numbered metric weights.
Who can remember 355ml of soda? Most can remember a 12 0z. can. But conversely, we can all easily visualize a 2 litre bottle of soda pop and none of us call it a "2.1 Quart" bottle, simply because it was made in a new package with an even/whole number.
If things were introduced in sizes that made sense and were easy to remember, they could implement the metric system a lot quicker.
