At some point someone will stumble along here and point out the thread that has a response perfect for this. I'm not going to do that though.
Why pairings? Well I personally am not a fan of only listing pairings. However, even when I put out a fully formed plot, or even just a bit of musings of a plot, it can be broken down to role a x role b. Got a super awesome plot about something or another? Well you'll have your role, and the one you want your partner to fulfill. Most 1x1s don't have a wide array of PC's. I know personally, my partner and I will use NPC's other than our main characters, but they aren't fully fleshed out characters. It seems to me the nature of 1x1s. If you want a larger cast of PCs, group rps seem the way to go. Alternatively, you can create your own int check specifying your want of multiple roles created by you and your partner. To me, that gets out of hand easily and I wouldn't be able to keep track of them all
As for why romance?
It's an easy plot device. Or its just plain what people want. It's hard to say why people want that, countless reasons I suppose. But its a reliable and typically, easily understood connection. It's why love songs are always on the radio. It's why romance novels survived the economy and e-books trend. It's why...well regardless of all that. It is possible to find a 1x1 sans romance, it just takes more searching or creating your own int check, again specifying whether you're ok with romance in that plot or not.