Popularity might be one reason, but it's also the air around that person I think. Kawamuu is seen as someone who works incredibly hard - that's one of the reasons why she became Anju's sub-leader. In the little talking part she said she practiced every day for that performance, renting a studio from 7am in the morning. The other reason might be that she not simply covered the song: She made it her own, while still showing a different side of herself. In total it was a really well constructed performance that added something new to an already popular song.
I was also surprised that some well done classic idol performances, like Ruru, Moe and Yuhane, weren't in the top 5, but Kawamuu is such a nice surprise.
Also, I think when it comes to girls like Masaki, Sakura and Sayuki, we might be used to them doing well already. (I do think Sayuki fell a bit flat here though, she sang well but didn't really deliver the feeling of the song.) The element of surprise is definitely another point. Regarding this, the bigger surprise is that Riko didn't make it into the top 5