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Most enemies don't show up more than once or twice.

Even those that do return often have to wait several years to do so. In the case of the Macra, the wait was almost exactly 40 years.

The Slitheen might not have been on Doctor Who for a while but they have been in two of the NSA books and four episodes of The Sarah Jane Adventures.

Also, they haven't been very popular with the audience.

Not to mention the slitheen were a family of alien criminals who were all killed in their second appearence except for one.

Above all, though, the Slitheen were creations of the previous showrunner, and Steven Moffat has tended to bring back Davies' creations only sparingly (an Ood in the Doctor's Wife, the Judoon cameo in "A Good Man Goes to War"). If a storyline supports bringing back the Slitheen, I'm sure he will, but they don't seem to fit the show's overall darker tone in the Moffat era.