Both "expansions" (I see them more as DLC, though that's basically saying the same thing and it's all semantics), while they add plenty of content in their own right, still aren't as substantial as Enemy Within was.
Combined, Alien Rulers and Shen's Last Gift may be around the same amount of content as Enemy Within, but it lacks the cohesiveness of a single expansion pack which I feel is one of the most important parts. Both just added some new things which were cool, but didn't feel like they really changed the game. Enemy Within was just... better.
That and I'm not a huge fan of any of the three DLCs. Anarchy's Children just does not fit with what I expect and want in aesthetics at all. Alien Hunters changed gameplay too much in what I personally feel in a negative way and made Alien Rulers just too powerful and random; some may like this but I definitely don't. And Shen's Last Gift doesn't really sell the SPARKs enough - they're kind of mediocre. Though Shen's Last Gift's mission was pretty fun to play through.
It was already a long shot that we would have an EW-type expansion, but as MSH pointed out, we did have a similar-ish cycle in Enemy Unknown. But our DLCs this time around are much bigger than Slingshot.
And @Neonivek's season pass comment:
That doesn't really have any relevance since it's not unheard of for companies to have "temporary" season passes only covering a portion of content or a limited amount of time. They're called "Season Passes," after all. Not "Infinite Passes." Firaxis was very clear on what the Season pass would entail, even going as far as to provide a brief summary of each DLC it would cover (Anarchy's Children, Alien Hunters, and Shen's Last Gift) before the base game was even out.