The recent pairing of Rey Mysterio and Sin Cara has not be done accidentally or lazily. New reports coming out now are that the WWE wants to build to a Sin Cara vs Rey Mysterio match at WrestleMania 29, but not before they have a lengthy run with the WWE Tag Team Titles.
There are other factors at play here as well. Rey Mysterio, despite appearing otherwise, is definitely not at 100% & tag matches give ample opportunity to hide that fact. Also, as I’ve stated in my past show reviews, Sin Cara isn’t connecting with the fans anywhere near the level of WWE expects him to be able to. They are hoping that getting the Rey Mysterio seal of approval provides enough of a rub for the audience to become invested in their breakup and subsequential feud.