This is mainly due to Sequel's superior connection pool design.
Sequel was designed to use a connection pool from the very beginning. It doesn't checkout a connection from the pool until it is needed, and returns it as soon as it is not needed.
ActiveRecord checks out a connection when it is needed, but you need to check the connection in manually, it doesn't do so automatically.
Sequel is safe to use in any threaded code, whereas ActiveRecord requires you to handle the connection pool checkins manually if used in threaded code.