I guess the issue is that they have to come up with a design method in which we only get a glimpse of their true form. Also there is the issue that people don;t seem to like stories where we don't get to kill someone/thing and this will always be the case with old gods. Also relates to the first point and maybe this is personal opinion but there has to be something there to explain how we mortals have the power to defeat an old god (unless they arn't as powerful as I perceive).
I think the Old Gods will always be more of a sub-plot, side quest matter, rather then the main focus.
The same way that N'zoth was involved with Deathwing, Yogg-Saron was behind Ulduar and Y'shaarj in Mists of Pandaria, none were major focuses of their respective expansions, but they still had enough involvement to be interesting and engaging. I think an expansion totally based on them would be rather dull.
Old Gods are exceptionally powerful, and killing off the remaining few is going to prove very challenging. I don't believe we know where most are, and I don't think it's possible to fully destroy them with our pathetic and weak mortal weaponry. It'd be a bit of a slap in the face for us to be able to overcome these immensely powerful beings in a mere two years, or when the next expansion launches and we're suddenly battling against far weaker foes.
If I recall correctly, the underwater raid featuring an Old God in Cataclysm (http://www.wowwiki.com/Abyssal_Maw), was supposed to theme Cataclysm as an Old God expansion as well. However due to lack of time they skipped this raid for now. I am unsure what was supposed to be inside, but I wouldn't be suprised if it has something to do with N'zoth.
While blizzard said it was a time issue, I'm almost 100% certain it was really a "people complained about how bad Vashj'ir was on a level not seen since the argument over the removal of the 3.0 raid meta drakes."
People HATE that zone. Even Blizz is willing to admit people hate that zone. An entire patch of nothing but underwater stuff would have been shooting themselves in the foot.
Personally, I would love to see an old god themed expansion. Provided Blizzard gets some better writers. H. P. Lovecraft was so skilled and subtle a writer that people thought his stories were real. With the old gods based so heavily and obviously on his mythos, I would hate for the hack who writes such one-dimensional masterpieces as Garrosh, post-5.0-Jaina Proudmoore and Varian chynn to work on the lore of such an expansion.