This isn’t a recent decision, which the title implies. This rewrite started in 2020, and they released Arti 1.0 in 2022. Check out the release post (https://blog.torproject.org/arti_100_released/) where they explain their rationale for the rewrite. They were unhappy with the state of the C codebase and couldn’t see a way to slowly refactor it. Their experience with Rust was positive for all the commonly cited reasons - if it compiles it works, good ecosystem leading to development velocity, better portability across operating systems, and attracting more contributors. They did say they weren’t happy at the time with binary sizes.

The change log in the arti repo (https://gitlab.torproject.org/tpo/core/arti/-/blob/main/CHAN...) shows a lot of recent development too- versions 1.6, 1.7 and 1.8 were released in the last 3 months and they talk about setting the foundations for larger features to come. All in all it seems like the decision worked out for the team.