The digital version of Pandemic suffers a lot from the "foot in both camps" issue, where the mobile version is too unwieldy, the PC version is too simple, and neither is nice. I still had fun with it, and the achievements were good at pushing me to find synergies rather than just stick with my favourite characters.
If I had to speculate, I'd imagine that doing justice to the franchise - adding multiplayer, adding a first class UI, or providing better support for all of the emerging offshoots - just wasn't feasible using this code base.
It's weird that they pulled it for sale only six months after releasing new DLC though. Maybe it is a licensing issue.