Because he's only been in the company for a year and a half? There's no mandate that says you have to win championship gold within a set date of being in the company. There's more talented wrestlers than Ryback who never won titles during their entire tenure in WWE, or if they did, it took them years to do it - Roddy Piper, Jake Roberts, Ted Dibiase, Rick Martel, etc.
Thus far, it hasn't made any sense for him to win any gold. He wasn't beating Punk. He wasn't beating Cena just a month or so after Cena got his biggest accolade ever with defeating The Rock. Curtis Axel was the new Paul Heyman Guy and was given the IC Title to correlate with that push. Dean Ambrose was given the US Title in order to further prop up the stature of The Shield by having them carry around gold. Same with Rollins and Reigns winning the tag titles. There were rumors of Ryback winning the WHC from Big Show at Wrestlemania, but those were either just rumors, or the plans were dismissed when Show was forced to drop the belt because of a contractual issue and by then, they had interest in building up Del Rio as a babyface.
I think there was a reasonable chance though that Ryback might have won the Intercontinental gold from The Miz around this time last year if he hadn't been rushed to the main event picture. If you wanna make a comparison with Goldberg, this is how his booking went as well (since they were clearly using Goldberg's push as the blueprint for Ryback) - in between squashing jobbers and low level talent and winning the world title from Hogan, they had him defeat Raven for the United States belt in what was actually a pretty hot feud. Could have done a similar thing with having The Miz as a cowardly IC Champion who tries to weasel his way out of fighting Ryback (or getting himself counted out/disqualified) but eventually, he gets squashed and drops the title to him.
