Why the decision to work with Shadowrun Returns and not one of the later games like Dragonfall and Hong Kong?
The super short answer: because it has got more options for creating new content.
The slightly longer answer without getting too technical is that all three Shadowrun games load assets from a big file with lots of game assets, and from a bunch of smaller files with smaller sets of assets. A modder with the Unity game developing tool, the same used to develop the game, can create more from these smaller files called bundles to create new character models, animations, outfits, item models and more. However, the big asset file is just too big to replace and it contains things like icons and fonts that can only be replaced (overwritten).
In Dragonfall and Hong Kong the developers decided to include outfits and item models (and some other stuff) in the big file, and the game will load those from there. That killed the ability to create new outfits and item models without overwriting some stock ones.
Hence, the decision to stick to Shadowrun Returns because it has got considerably greater freedom for content creation.