Tradition? How’d it start? Vast conspiracy? Who’s behind it? Alien invasion plot? How does that make sense?
Why are they in crates?
Situations like this almost always are marketing ploys that worked (per my gf, who is in advertising). I’ll ask my local grocery store produce dude when I stop later. You’ll likely have an answer by then, though.
I always thought it was so they don’t roll around.
When we buy them, they’re in a mesh-plastic bag, inside the aforementioned crate. You could eleminate the crate, and the bag of oranges still wouldn’t roll around.
When we go through checkout the cashier always asks us if we want to keep the box or not; we always say no.
Here we have some plastic mesh just stapled over the top of the crate so the Clementines don’t fall out. They do anyway. There’s no mesh bag.
Come to think of it… I suspect the mesh is to prevent people from adding more and more clementines into a heap in the crate more than to keep the roundlings from escaping.
ETA: Pic of box with mesh just overtop.
My apparently dangerous kitchen is full of round fruit: apples, oranges, grapefruit (go banana!). What makes clementines so special? Peaches are subject to rolling too and suffer worse in a fall, but you don’t see them boxed up.
I’ve always assumed it’s because they’re a bit fragile. They peel very easily, and the weight of a standard case or sack of fruit is probably more than enough to start splitting them open.
Putting them in those mini crates provides some stacking framework so they’re not crushed.
Digging in Google finds the real reason for the crates - it’s clever marketing.
Originally, clementines were sold loose. People would buy them onesey-twosey and stores were left with piles of oranges not selling. Back in 1990, the marketing director for a growers’ co-op tried packing them in 5-pound boxes with gift bows. They started flying off the shelves, and the rest is produce marketing history.
Isn’t it obvious that they’re sold in crates to appeal to people like me who use the crates as kindling? No? Really?
Genius.
Obviously, Clementines are sold in crates because only their feet will fit in #9 shoe boxes. 
The guys at my work developed quite a craving for the clementines a few years ago. They do make a handy snack and you can go through the small crate in not too much time. I still spot a crate or two in various offices.
You almost made me laugh out loud at work and my boss is sitting right behind me :eek: He’s working while I’m reading SDMB .
When I visit my parents in Western Canada for Christmas, they have proper Satsumas (usually just called “mandarins”) around Christmas time rather than Clementines. The Satsumas are usually sold in one-kilo or two-kilo cardboard boxes, with each fruit individually wrapped in tissue paper. No bows, although the boxes are usually printed with colorful designs and/or pictures. Unless my later experiences are seeping into my earlier memories, such boxes predate 1990.
Oh yeah
I can find the Satsumas for a few months each year and they kick the butts of the clementines.
And my take on the clementine crates is that there are always some crappy pieces in each crate and if people got to hand pick them the crappy ones would never sell…actually sometimes my Whole Foods has loose clementines, the price per pound works out to more than the crate price though. I do that if I can’t find Satsumas.
Oh my darling, oh my darling,
Oh my darling, Clementine!
Thou art lost and gone forever
Dreadful sorry, Clementine …
whut? 
There is no mesh bag here. Just the crate.
Where I live, what we used to call “Christmas” oranges come in sturdy cardboard boxes that can be stacked high without damaging the contents as another poster described. It wasn’t very long ago though that wood orange crates were standard (ok, 30-40 years ago). They didn’t look much like the pictures that were linked but I wonder if there’s any nostalgia marketing involved in using wood where cardboard will do.
I thought it was so you could make a playground for your pet mice once you’re done wiith the clementines.
No?
Well, I set aside my little wooden crates because a) they’re “too good to throw away” and b) someday I’m going to make…something… rustic and crafty…with them.
I tried finding the answer on Darling Clementine’s (a distributor) site. Didn’t find it, but did find this page of crate ideas.