The main issue I think was the RTWP crowd. They made a real big fuss about turn-based combat being a sluggish drag, so Larian's first initiative was to create as dynamic and swift of a combat system as they could, so they erased all the features that would slow down combat (reactions being the main victim here).
I do think that at the end of the development progress, they will see that reactions are pretty vital for the 5e system and will include them.
