He found that the environmental impact created by the washing of the glass bottles was worse than the impact of the entire production and disposal cycle for the cartons. If you added in the production of the glass, the recycling of glass when it broke, and the extra impact from transport (less space due to not being able to pack as well, heavier) there was no competition at all - glass was way, way worse.
Plastic was a bit better than glass, and carton was the best available option. So they stayed with carton.
This was ~30 years ago, mind, so the equation may have changed. But I still find it important to check before deciding "Let's go glass" is the right option.