It's a set of tradeoffs. The 6oz bottles were (I think) returnable. Even better than recycling, they only needed to be washed/sterilized to re-use. I'm old enough to remember Coke and other brands in returnable bottles up to 16oz.

They were heavy. The transportation costs savings with the lighter plastic bottles (combined with maybe the raw materials savings, and savings not having to transport, handle, wash, and sterilize the emptys) made plastic more economical than returnable glass.

Manufacturing cost for glass bottles almost certainly higher than plastic. You have to melt either sand or recycled crushed glass which takes a lot of energy.

Some jurisdictions do have returnable plastic bottles. They are much heavier than the disposable ones but still lighter than glass.

Interestingly for some reason beer in plastic bottles has never caught on, it's mostly sold in glass bottles and cans.