The main reason they didn't bring them forward is because of the minority of weapons EG fatebringer and ghorn were being used by the majority of people even when they didn't meet the new light standard (TDB), The problem with bringing them through would be that because everyone loves the year 1 gear so much because of how nice it was to use and how good the perks work together they would use nothing else and bungie wish people to use the new weapons they design.