Restic is one of the best backup tools ever written. It's free, open-source, fast, and its deduplicated, content-addressable repository format is genuinely excellent. If you're comfortable on the command line, it's hard to beat.
BackupKit is for the much larger group of people who want exactly what restic does — encrypted, scheduled, owned backups — but don't want to write cron jobs, manage a repository from the terminal, or memorize restore syntax under pressure. Same philosophy, different surface.
This is an honest comparison. Restic is the better tool for a certain kind of user, and we'll say so.