Pure guesswork of course, but it occurred to me to wonder. Whether it's because the required favor quest is just not worth the effort, or because s/he's bland or unpleasant or just simply ugly, or maybe it's not even noticeable that s/he CAN be married. Obviously this is not an absolute, there's somebody out there for everyone, or so they say. http://www.uesp.net/wiki/Skyrim:Marriage, halfway down, is the list of everyone available.
For females, I'm going to guess it's Temba Wide-Arm. She's grating and gruff, even after marriage, or so I've read. I recall a few years ago someone posting that they married her just for that reason, and hey, whatever you're into, but that's the only time I've ever seen anyone post it.
For males, my guess is Filnjar, the blacksmith at Shor's Stone. I didn't even know he was a candidate until I just so happened to talk to him with an equipped Amulet of Mara after clearing the mine. (It's not an extraordinarily useful amulet most of the time anyway.) He's neither interesting nor a particularly good looking guy in my opinion, and with Shor's Stone as out of the way as it is he just doesn't strike me as someone you'd pay much attention to unless you're unloading loot.