This may be a stupid question, but consider the source. 
Why are most flannel shirts done in plaid? A multitude of colors, to be sure (one of my favorite shirts is a bright plum with grey) but…plaid.
I love flannel shirts, they are a perfect toss on when you’re chilly, but they’re… plaid.
Why?
No idea honestly. Tradition is the only thing I can think of.
Because you can’t be a lumberjack in a paisley flannel shirt. Duh!
ETA: Reversed comments as this is GQ. Sorry bout that.
Can’t answer the question, but can sympathize. I love flannel shirts, but greatly prefer solids to plaids. As the previous poster said, it’s probably just a matter of tradition.
Interesting though, that chamois shirts (chamois by definition is just a “very heavy, dense cotton flannel”) nearly always come in solid colors.
[shrug]
SS
I have quite a few solid color flannel shirts. It’s possible they’re Chamois I guess, but I don’t think they were called that in the catalog.
Unix requirement.
I wonder if you just notice flannel shirts more that are plaid.
You see a plaid shirt and think “Oh that must be flannel.”
You see a purple shirt and it’s not evident without touching it, or at least seeing it very close, that it’s flannel.
I don’t know the answer, but I have a houndstooth flannel shirt that is one of my favorites.
And I agree that flannel should be available in different patterns.
For once I don’t think it’s confirmation bias. Just look at this: Google image search
It’s the color of the dye.
http://www.hickorees.com/CATEGORIES/SHIRTS/BRANDS/FOLK/PRODUCTS/Flannel-Raglan-Shirt-Peacock-Limited-Edition
Also, search for ‘non-plaid flannel shirts’ for other places that eschew plaid.
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Ah, but the OP did ask why MOST Flannel Shirts are plaid, acknowledging that in fact there are non-plaid flannel shirts to be had.
So far, research on the web indicating that a) ‘Yes, most flannel shirts are plaid’ (Caveat - this applies to the US only) and b) Nobody seems to know why besides tradition and rugged fashion and all that.
Were the majority of flannel shirts prior to, say the late 1960s not plaid? (a work-like blue, perhaps).
To distinguish shirts from pyjamas made of the same material? http://www.rivers.com.au/catalogdb/blowuppage.asp?SP=22595&PS=14 is my favourite flannie of all time and I have loved hundreds over the years.
Proof that if you love too much, you’ll go blind.
WHY did I click on that link?
That’s not so bad.
If silenus ever posts a link to a favorite shirt, wear dark glasses. 
I wore it to my city office job once where I am the token middle aged bull dyke, they thought it was an arty statement and I was told it was awesome all day long.
I say it’s tradition. I can’t say why exactly. I have a vague feeling the original fabric that was woven in a plaid pattern to differentiate tribes/clans was later duplicated in what we now know as flannel, simply due to tradition. Flannel is cheaper, lighter, suitable for manly men doing manly things. It’s plaid because…a lumberjack wearing a mint green or lavendar flannel shirt would just look silly! And it would show the dirt! A plaid flannel shirt stands out when you are looking for it; a solid color shirt may or may not be flannel, you would have to feel it and look at it because it could be wool, cotton, courderoy. So plaid it is…I love the Stewart plaid above all others, and I look for flannel pajama pants in a Stewart plaid. You can wear any color top with them, any color at all.
Thanks for the input! It’s obviously not a world-altering question, but just something that makes me scratch my head and go hmmmm…
There are few things I love more than a nice soft flannel shirt to ward off the chill… Just trying to unravel some of the mystery!
I don’t have time to search for it now, but a while back there was a thread asking why school uniforms so often involve plaid. Most of the reasons people came up with would probably apply to shirts meant to be worn while doing physical labor in the outdoors: patterned cloth doesn’t show dirt and stains as much, patterns are helpful in distinguishing otherwise similar garments*, and if you want a fairly cheap but durable patterned fabric you’re limited to patterns that can be woven into the cloth like stripes, checks, and plaid. And, as others have mentioned, there’s also the element of tradition.
*For school uniforms the distinction would be between schools rather than individual students, while for lumberjacks, etc., it would be more Pete’s shirt vs. Jim’s shirt. If a group of workers are camping out together there’s probably plenty of opportunity for clothes to get mixed up, and distinct patterns may help workers to easily identify each other when they’re some distance apart.
I thought school uniforms involved plaid because they are derived from Scottish traditional clothing.
There’s an episode of ER where the doctors are discussing that their chief of surgery arrived in the emergency room dressed up like a Catholic schoolgirl. Then we see the second doctor go in to talk to him, and discover him wearing a kilt and Scottish fare. Well, a plaid skirt and knee socks, a white shirt, and a little purse hanging from the belt - that looks like a Catholic school girl outfit, too.
I suppose that’s possible, but why would schools outside Scotland want to use uniforms derived from Scottish traditional clothing? Scottish immigrants could be a factor in the US and Canada, but I used to work in Japan and plaid school uniforms are fairly common there too. Deliberately going for a Scottish look seems like a particularly odd choice for Catholic schools outside Scotland, as Scotland is not a predominantly Catholic country.