Why is that more likely? To me it sounds reasonable that small countries have tradition in one of the genders only. Not an expert in biathlon, but in Cross Country that’s definitely the case.
Just in recent years, though, you could have had potential medal contenders out of Slovenia (Jakov Fak & Teja Gregorin), Latvia (Andrejs Rastorgujevs & Baiba Bendika) and Austria (Simon Eder or Dominik Landertinger & Lisa Theresa Hauser) who would struggle to be competitive in either a one-gender two-person competition, or in a conventional relay. For many years the Austrian men were good but the women lagged well behind, now they've improved the women but most of their most successful generation retired on the men's side. Even among those, when they were active the Slovenes were pretty competitive as a Mixed Relay unit with Fak, Bauer, Gregorin and Andreja Mali.
As outsiders who could be competitive in this format you could have also added Estonia (Rene Zahkna & Tuuli Tomingas), until recently Lithuania (Tomas Kaukenas & Diana Rasimoviciute), the imported Koreans (Timofey Lapshin & either Anna Frolina or Ekaterina Avvakumova) and even Japan (Michito & Fuyuko Tachizaki).
Of those who would reasonably be better in one-gender versions of the Single Mixed Relay I'm not convinced there are any at the moment. Maybe the Czechs if Michal Krčmář continues to struggle to show any consistency, as Jislová had a good season, but the competition being based mainly on fast shooting and relatively short skiing loops doesn't suit Davidová. Slovakia maybe with the Fialková sisters, or Poland with Monika Hojnisz-Staręga and either Kinga Zbylut or Kamila Żuk, since their men have really disappeared since Tomasz Sikora retired. Maybe the Finnish men with Tuomas Harjula and Tero Seppälä? The American men ten years ago with Lowell Bailey & Tim Burke, and more recently their women with Susan Dunklee & Clare Egan, would be a possibility, but as all four are now retired...
The other thing to bear in mind is that adding this to the Olympic program will probably change the Mixed Relay startlist as it is unlikely many athletes will do both, so some teams will target one race and some teams will target another, based on their athletes' skillsets.