Romance isn't the entirety of 1x1, but it's a hefty majority for sure. I just looked at the 1x1 Interest Checks section and just from the title alone 17 of the 30 on the front page were blatant romance pairing things, and after looking at the other ones it seems only 3 threads out of the 30 were not wholly or partly romance-based interest checks.

I'd guess that it's partly a product of the nature of a 1x1 RP and partly due to the reputation building on itself. If someone wants to do some kind of romance RP, odds are they're looking for it to be one on one, as group stuff generally has some goal in mind that doesn't allow for many romantic interludes without derailing the whole damned thing. People who like to do romance RP stuff naturally gravitate toward 1x1 because that's where they'll be most likely to get what they want. After enough time and enough dedicated romance RPers using the section they became an apparent or actual majority; if the former, after enough time of people perceiving the 1x1 section as being about romance and thus going elsewhere if they aren't into it, then it turned into an actual majority. More time and more reinforced perceptions lead us to today, where romance is a strong majority of what goes on in the section. Or hell, I could be wrong and it started off that way entirely, but either way it seems to me a rather natural state of affairs.

Also, there's something to be said for the idea that those seeking romance are more likely to make their own interest check threads than the average non-romance 1x1 user. People have all sorts of particularities when it comes to romantic inclinations, with certain setups and scenarios they're interested in (and if they go for the X-rated side of things, kinks and turn-offs also come into play), so they're likely to make a thread outlining exactly what they want rather than combing through everyone else's things. This hypothesis of mine is backed up by the numbers: there have been 909 1x1 Interest Check threads made, and the next highest is Casual at 834, and Casual is generally viewed as the most active section (which is backed up by that section having by far the most non-interest check posts of any section). The interest check area is the face of a section on this forum, and since that's so heavily biased toward romance, well, I'm not surprised that it's hard to find other things. I know I avoid the 1x1 section because I'm not interested in romance as a primary focus and the few times I bothered to look that was all I found, and I wouldn't be surprised if there are many others like me.

It's unfortunate for those who wish to do something else in a 1x1 RP, but I can't think of much of anything that can be done about it.