Memcached exists since 2003, while Redis was created in 2009. This means memcached had 6 years to get adopted and developers to develop expertise with it. Since they both do the job, those who already know how to operate Memcached don’t see much reason to switch to redis. Same happens with sidekiq/resque.
Stick with Redis, since that’s what sidekiq uses, and then you don’t have to learn two very similar pieces of software.