I think in a lot of cases people are going for the "Everyone has someone" ending and need a main character to stick with Ukyo. Konatsu and Akari were late additions and don't really play a substantial role in the story beyond their introductory arcs, so there's no perceived obligation to hook them up with someone or otherwise involve them. The 'big six' that need to get hooked up are usually Ranma, Akane, Ukyo, Shampoo, Mousse, and Ryoga.. sometimes with Kasumi, Tofu, Nabiki, and Kuno thrown in for good measure.
Also, there's nothing immediately available to write about regarding Akari.. if Ryoga goes with her, there's no conflict to resolve unless the author creates it from whole cloth (i.e. wierd pig attitudes or other character flaws). Whereas to hook up Ryoga and Ukyo, a lot of legwork needs to be done -- there's a story there to write, even if it's been written forty times already.