The Left hates plastic because it comes from petroleum---and petroleum continues to kick the ass of every one of the Left's pet "alternative energies."
They can't ban petroleum yet, though they're itching to---so they wage war on plastic. To that end, they babble about how "plastic lasts forever," or some such nonsense. Anyone who has ever owned anything made of plastic knows that this is ridiculous; plastic breaks down, as you know if you've ever had a switch snap off in your hand, had a plastic housing crack without warning, seen your Tupperware get greasy and start to melt of its own accord, watch what happens to some plastics when they're left out in the sun too long.
Plastic is less permanent and stable than its predecessor celluloid, and is certainly less permanent and stable than the bronze, brass, iron, earthenware and wood used by pre-modern civilization. Note, by the way, that were it not for these durable materials our archaeological record of prior civilizations would be much the poorer; is it likely that our easily-shattered, readily crumblable plastic artifacts will last 3000 years as the detritus of the Assyrians did?
The War on Plastic is merely a proxy, a micro-war for the Left's War on Petroleum.