You're absolutely right, I could have included more historical information about the onigiri, and maybe that would have added more value to this post, I am always conscious about the length of my posts, though, and I wanted this post to be a How To post that highlighted the design of onigiri packaging, which I think is incredible, in a secondary sort of way.

Onigiri really is and has been a staple food in Japan for a long time. Using only two sources to gather this information, http://www.onigi-re.com/knowledge/history/ and http://www.o29riclub.com/knowledge/history.html (mostly the latter), I can tell you this.

There is fossilized evidence from Ishikawa Prefecture that suggests onigiri have been eaten in Japan since around 100 AD. Up until relatively recently, maybe 100 or 150 years ago, onigiri were considered to be something of a fast, portable food. It could be taken anywhere and eaten anywhere quickly. As such, it was a food for soldiers, field workers, and travelers for a long time. Nowadays, onigiri are thought of as an everyday food to be eaten and bought at home, at restaurants, on the go, etc.

Prior to the mid Edo Period, around 1600, onigiri were apparently wrapped in bamboo leaves. I don't know what economic conditions prompted the transition to nori, but according to the websites I referenced, people began wrapping onigiri in nori for two reasons. One: nori has a high nutritional content. Two: nori is both edible and it keeps the rice from sticking to a person's hands and fingers.

In addition, onigiri hold a special place in many old Japanese folktales, the versions of which change from place to place. Also, the first 'Eki Ben' (station bento), which began being sold in Utsunomiya Station in 1885, was an onigiri bento that included two onigiri that were flavored with sesame seeds.

I hope this comment satisfies your curiosity a little more.