I have read stories of Activision (and Blizzard, which they own) cutting back and saving money. Perhaps the retro collections don't sell very well? Or not enough to keep up with the maintenance demands of the mobile store?
I'd be surprised if one of the world's largest entertainment software publishers would nix a collection like this purely because of financial belt-tightening. Code Mystics already has excellent emulation of the Atari 2600 (and Atari 5200, too) up and running on current platforms. They just need to whip up a new user interface, and...ahem..."patch" in a suitable system of tracking the player's accomplishments.
All of the versions of Activision Anthology (a.k.a. Activision Hits Remixed) were extremely well done, on every platform they were released on. It would be awesome to see new and improved versions for current platforms. The accessibility through modern digital download stores would enable this collection to remain relevant for years and decades to come.