UL and durable aren't always compatible goals. There are limits to material engineering and manufacturing techniques. Without any specific inside information, I would suggest that manufacturers of ultra lightweight gear are finding that buyers are ignoring what should be the obvious relationship between weight and durability and expecting a UL pack to last as long as something made out of Cordura. Or, they find the packs being used in ways for which they aren't designed, such as stuffing too much gear into them or bushwacking with lightweight outerwear than tears easily.

When they get complaints or see repeated failures of materials or manufacturing techniques, they make changes and those probably add some weight. Maybe not much, but enough to overcome the problem.

Retaining brand loyalty is important to small manufacturers as well a big ones, so adding durability is a choice that some have apparently determined is more important that saving a couple of ounces.