
Well here's a big one - a lot of plastic bags are so flimsy they don't last very long. More than once I've brought my light shopping home from Somerfield, only 300 metres down the road, and by the time I've got in the bags have been coming apart, with growing holes in the side and sometimes even the bottom. No way am I going to try reusing such bags. And they certainly wouldn't last in the usual scrum on the bus when I go to Asda to do my main shopping.
Also there's a deep social reluctance to be seen in one store with carrier bags from one of its rivals. This is especially the case for supermarkets, where it's very rare for someone to visit two during a single shopping trip. Now you can say this is just human cowardice, but store security guards taking an excessive interest don't exactly help encourage people to recycle bags.
Perhaps the solution to both of these is the creation of very sturdy (including bus proof), reusable plastic bags that are not store specific that can be easily used at any store time and time again. It would be a significant change to the way they're distributed - heck sometimes at Somerfield it's a problem to even get a single bag off the dispenser without it tearing - but until reusing bags is a real workable solution it's going to be very hard to get people to switch away from new bags each time.