Because I'm the kind of person who notices trends like this. And then compiles them into lists.
-The most popular couples are Church/Tucker, Grif/Simmons, Caboose/Donut, Church/Wash, and, on the het side of things, Church/Tex.
-Tex/Sister is also fairly popular, but I suspect that's because it's the only even remotely viable femmeslash pairing.
-Other common couples include Church/Caboose, Sarge/Caboose, Doc/O'Malley, and Sarge/Grif
-Not common, but notable purely for the fact that they've been done more than once: Tucker/Crunchbite and Simmons/Sheila.
-Notable for its lack of popularity is Sarge/Simmons, especially given certain lines in the 3rd episode of Season 3.
-Unsurprisingly, given that he appears in 3 out of the 5 most popular couples, Church is the fandom bicycle. To the point where "Churchslut" is a common term.
-Grif is a close second.
-Church/Grif, unsurprisingly given the above, is certainly not an unpopular pairing.
-I've noticed at least 3 fic based off their time in prison together. I have not paid enough attention to observe whether or not Church usually actually is the bitch.
-Other particularly popular situations I have seen written at least thrice: Grif/Simmons "What were you doing parked in the shade for two hours?" "Well, I can tell you what we weren't doing...." (I kind of
partially blame Gus Sorola for this.), and, oddly, Church/Simmons based on the brief time in Season 4 where they were the only inhabitants of Blue Base. With varying degrees of consent on Simmons's part.
-Actually, I think the fic for this fandom as a whole could do with a good bit less rape. :|
-No segue here, but there is a large trend of denying that Grif is fat, claiming that he's "just unathletic".
-More bizarrely, there's also a slight trend not only defattifying him, but dehumanizing him. Seriously, there are, like, 3 different series that involve Grif being something other than human. And it's always Grif, never anybody else.
-Also popular are High School AU's.
-Interesting is the total lack of genderswap and other popular AU types, though.
-There are, however, a number of fics featuring chance-or-not-so-chance encounters after the end of the war.
-While "Grif is Hawaiian" has managed to work its way into general fandom knowledge, despite being DVD profile exclusive content, "Simmons is Dutch Irish in nationality as well as ethnicity" has not had quite the same degree of luck, and even the people who know about it can't agree on what that actually means. I've seen Simmons having origins in anywhere from Boston to New Orleans to just Ireland to one particularly humorous example that I kind of wish I could find again that explained Dutch Ireland as the result of Ireland suffering from a huge economic crisis while the Netherlands happened to be gaining an empire, prompting Ireland to decide that their best bet was to surrender themselves up to the Netherlands. The possibility of Dutch Ireland being a colony world doesn't seem to have come up in the realm of fic, sadly.
-Common colorations: Grif with specifically hazel eyes. Simmons with red hair and either green or blue eyes (possibly ignoring the Dutch part of Dutch Irish like above). Caboose and Donut are both blue-eyed blonds. Church has black hair and blue eyes and usually a goatee. Tex is a redhead.
-Everybody in Blood Gulch is incredibly pretty. Everybody. Except Sarge. Sometimes.
-Simmons being a cyborg, Grif having his leftover parts, and Church and/or Tex being robots are often blatantly ignored. When Simmons being a cyborg does come up in the context of description rather than as a vehicle for angst plot, it tends to just be the left arm and maybe an eye. Disorientingly, sometimes Simmons being a cyborg will be included, but Grif having his leftovers will be left out. When it is included, mismatched eyes are, again, popular. The matters of the ass and balls are strictly never brought up. Church and Tex are usually fanwanked into being incredibly life-like robots.
-While I understand that a number of fics out there were written before Sister even debuted or alternatively before the Season 5 DVD came out and revealed her actual name, I still find it seriously disorienting to see a siblingless Grif in fic or Sister named anything other than Sister or Kaikaina. Bizarrely, K names seem popular for her.