So pretty shocked that this isn’t answerable with a 10-second Google, but its not. I get they are traditionally Polynesian, specifically Hawaiian (or at least that is where the US tradition originated).
But that doesn’t answer the question how did the Hawaiian traditional gift of a garland of flowers become so synonymous with graduation all over the US?
Do you have a cite for it being “so synonymous with graduation all over the US”? Because I’ve been to a number of graduations in the Midwest (grade school, high school, and college), and I’ve never seen it.
I’ve attended several in California and the East Coast (DC/Maryland), both college and high school, and its been as much part of the garb as mortarboards. Never seen it outside the US (though graduations, particularly for school rather than university) are much more of a thing in the US than elsewhere.
So, your cite is personal experience. I think, then, that you really should be asking two questions:
Are graduation leis as ubiquitous a thing as I think they are? (I suspect that the answer is “no, they aren’t,” but I’d be happy to be proven incorrect)
Well “Graduation Leis” are listed on the Party City “party essentials” site:
So its clear they are synonymous to some degree.
But it never occurred to me it wasn’t completely universal in the US (as I’ve attended a pretty wide sample of different types on graduation in different bits of the US, albeit geographically centered on the SF Bay Area and DC). Those who have never seen them, how recent are we talking about? Most of my mine have been friends college/grad-school graduation or their kids high school graduations, in the last 5-10 years.
It seems to be a relatively recent development here in Southern California. Never happened when I was in high school/college in the 80s. The first time I saw it outside of Hawaii was at my son’s middle school graduation in 2014. According to the census, my community is about 30% Asian, so I figured that had something to do with it.
But if Party City is pushing them, it could be spreading. Remember, 15 years ago no one had ever heard of a goddamn gender reveal party.
I’ve spent a few minutes doing Google image searches for the 2019* graduation ceremonies at my high school alma mater (in Green Bay, WI), at several high schools in suburban Chicago, at the high school in Alabama where many of my clients have kids, and at my college alma mater (University of Wisconsin-Madison). I found a mix of pictures both during ceremonies, and more “candid” shots from afterwards. I found exactly one lei, in one of dozens of pictures I found of UW commencement ceremonies:
You are mistaken. This is not a thing with graduations all over the US. In fact, I’ve lived in 5 different states in my lifetime and have never seen leis associated with graduation.
Yeah it seems this is the likely answer to the OP. Leis are (or will become) synonymous with graduation because people who sell you tat want to sell you more tat.
As I was reading the above comments I was wondering the exact same thing as @griffin1977: Specifically the order of the causality:
Does Party City push leis as an essential gift because people want them, or do people want them because Party City is pushing them as an essential gift?