To add to what throwaway2048 said, as well as having high performance requirements, they're small and don't change much. You wouldn't want to write your typical

To add to what throwaway2048 said, as well as having high performance requirements, they're small and don't change much. You wouldn't want to write your typical CRUD app in Rust or C++ because developer speed is more important than application speed. But bittorrent trackers, while using http, are operating to a strict protocol, and are nowhere near as large (in terms of features and code size) or as fast changing as typical web apps.