They wanted the community still using the original game to not be separated from playing with players in Reforged, which is a noble ambition.
I think they could have updated the engine for the original game right alongside Reforged, but that might have irrevocably altered how the game played which is a big risk. Wouldn't want to alienate the old community.
Sadly, it seems they've done just that without the benefits of a modern engine. A profound Lose-Lose.