Staffan said:
Probably to justify its price compared to a Bag of Holding, given the smaller amount of storage.
You also have to carry a bag of holding in your hands. A haversack is so much more... handy.
I've always kept the mea rule about these. My rationale is this : The sack doesn't accelerate the movement of your hand, it just makes what you are looking for be at the top of the bag. It takes a mea to draw a weapon and the movement to get something from the bag is somewhat equivalent. I did allow people to retrieve things with a free action if they had quickdraw, too.