That's because the long-standing notion was that Luke Harper was sitting out the remainder of his WWE contract, after asking for his release in the spring.
According to Dave Meltzer, the decision to bring Harper back in was a late one, because WWE is planning a tag team match, and Eric Rowan needed a partner. At some indeterminate future point (perhaps Hell in a Cell), the unified Bludgeons are penciled in to take on Roman Reigns and Daniel Bryan.
A report from PWInsider indicates that there was high secrecy in regards to Harper's return. His name was apparently not listed on the format sheet, and he was kept hidden backstage until just before his on-screen return, surprising his locker room peers just as much as the fans.
Harper's last match took place on April 9, where he defeated EC3 in a dark match at that night's SmackDown tapings.