Again this goes back to the Endings. "The ending is, as those who've played the game would know, final, with no possibility of a future for the Commander Shepard character. As such, a DLC like Leviathan, regardless of the quality of its content, may fail to interest anyone who feels as though their story has already ended", according to the article.
Additionally a fan on Twitter asked whether BioWare had any plans to develop single-player DLCs in the future. ME3 writer Patrick Weekes wrote: "Future DLC plans of any kind depend upon success of Leviathan. If we see that people want SP DLC, can always make more."
Update: User JakePT below commented that Mr. Weekes wrote on Twitter: "Hey, all -- apologies to people who took my note about future DLC with concern. If you want more, I don't think you need to worry. :)".
I guess it goes without saying that Leviathan did well :).