What is the reason that the materials of the lens barrels changed from metal to plastic?

Question

Most of my older lenses have a shell of metal instead of plastic that is common today on all lenses even the expensive ones (with the possible exception of expensive tele lenses). When I compare the feel of a newer Canon L lens with the L lenses of the FD mount the older ones seem to be a lot tougher. I realise that the plastic materials that the newer lens is made of is not of any plastic mold and it's very tough indeed but can it really compare to the older constructions?

I have a hard time believing that Canon would use plastic just to cut the costs with some dollars when manufacturing lenses that cost thousands of dollars.

What is the reason that the materials of the lens barrels changed?

Answer 1

It is not just the barrels of high quality lenses. The light boxes of the top tier cameras from both Canon and Nikon are now made of engineering grade composites (i.e. plastic). This includes models such as the 1D X and D4.

Why? Because those materials can be engineered to be stronger, lighter, and less sensitive to expansion/contraction with changes in temperature than the metal alloys used in the past.