Okay, it's late, I'm tired, and I've been into my cup a bit, but I will try to explain.

For one thing, from your own Hurstwic, the typical Viking-era shield was chamfered from about 1/2" towards the center to 1/4" at the edges. This means that it, in and of itself, is of dubious durability when struck repeatedly by weapons. While documents show that shields were constructed from two layers of linden, most existing shields are single layers of butted planks, including pine, spruce, and fir (relatively soft woods).

We don't have much archaeological evidence to support iron-banded shields from the Viking era. It's likely that the majority of shields were banded with rawhide and/or leather.

Shields were also likely covered in leather or linen to attempt to strengthen them.

Helmets, on the other hand, were expensive. A typical "Viking" may go on a Viking with two to three shields so that he might replace once that has been damaged, but replacing a helmet typically meant replacing the "Viking" as well. As such, you're very likely to find more of a semi-disposable (speculation, but it's likely they kept bosses and re-used them) item as opposed to a helmet that was only worn by those would could afford them.

Likewise with maille. as Hurstwic states, few could afford that much iron and the work needed in shaping it. Even making butted maille is an exercise in tedium, and setting rivets takes even longer. I have red that a maille hauberk would roughly cost as much as a farm, and as such only the most wealty northmen, or their retainers, could afford them.

TL;DR: Your average "Viking" had several shields, but only maybe a helmet and probably not any other body armor. As such, it's far more likely that shield bosses survived the ages.