Back when this was new, there were studies showing that the typical canvas bags sold at grocers are also breeding grounds for all sorts of nasty things that you don't want to be transporting your food in.
So it's just purely all downsides. Like security theater, but for the environment.
Tell that to the Anacostia river in DC! They great at reducing watershed pollution. It's really noticeable how much better things have gotten since the bag fee.
As a side effect, DC's water authority has also been able to cut maintenance budgets because clumps of bags were our main source of sewer clogs.
So it's just purely all downsides. Like security theater, but for the environment.