Aiba commented that he was troubled at first whether to accept the offer but when he thought of the fact that he might never get such a chance again in future, he decided to be the host as a challenge to himself rather than regret it every year in future. He pledged to work hard during the broadcast and asked for everyone's support. As for Arimura, she mentioned that when she first heard about the offer, she thought that this was something which would never come her way again in future. Although it is still taking her some time for the reality to sink in and that she's worried about that she can't handle such a huge responsibility, she will express her gratitude for giving her this chance by trying her best and enjoying the experience.
As for why they were chosen as the hosts, the director of NHK's entertainment programmes department Inoue Keisuke said that Aiba had shown his leadership skills and built up his knowledge of sports and hosting ever since he started hosting NHK's programme "Gutto! Sports" from April this year. As for Arimura, he said that she was selected for her fresh appeal as she has never hosted any programme before.
Sources: Nikkansports / Sanspo 1 / Sanspo 2