I think there's a few confounding factors.
One is that there's no infrastructure in a lot of these places. When you're in South Ease Asia, there's nowhere to put your rubbish, there's no rubbish bins, and there's no proper waste collection for street stalls, so your chuck your rubbish on the ground.
At least in the larger towns and cities in Vietnam, they have workers that tow around a big rubbish bin on wheels and collect the rubbish off the street, but in smaller villages, they just occasionally pile it up and burn it.
Lack of infrastructure is definitely a problem there.
But on the other hand, a lot of people just don't give a fuck. Maybe it's because the place is so dirty anyway. My friend was draining the petrol tank, and asked if there was anywhere he could dispose of the water contaminated petrol at the hostel or nearby. They just told him to tip it on the street. They were surprised that he wanted to find a better place to tip it where it wasn't just going to run all over the concrete, then into drains and the sea, not to mention the fire risk of dumping a couple of litres of petrol on the concrete.
I guess you get used to your surroundings. It's certainly not an ingrained culture thing that's peculiar to Asia; Singapore and Japan are both very clean, and I remember Kuala Lumpur being reasonably tidy too.