Alright, so you know the how, but what about the why?
Why do Netherweave bags sell on a consistent basis?
I've come up with a couple of reasons why this item is the best selling container:
1. The mats (ergo the price).
When compared to other bags, the Netherweave bags are the simplest to make as they require only the cloth + thread. Runecloth bags require Rugged Lether, and Imbued Netherweave bags and Frostweave bags require enchanting mats. Usually, less mats (or less unique mats) converts to less cost. Not always, but definitely true in this case.
And less cost means people are willing to make them.
2. Price per slot. The Netherweave bag price works out to be around 50 silver per slot; while smaller bags offer slightly better price per slot, they fall off in value in that they just don't carry as much. This is where the whole (the total bag space) is more valuable than the parts (the slots).
The larger bags have a much bigger price per slot, so it appears that the Netherweave bags are just big enough.
3. BoE. That's right, they're Bind on Equip. While this might not seem to be significant, it's huge for the market. BoE means that when little Johnny gets all jacked up about starting a Warlock and buys all Netherweave bags for it, and then abandons it at level 6...I make money, and Johnny sells the bags to a vendor for a gold each.
Then, next month, when Johnny gets all excited about rolling a Druid....well, I'm sure you can see the pattern. I like Johnny. I like him a lot!
So, there you have it. Availability, value, and consumption combine to create a steady, profitible market where I can make some coin!