They can't because the 1.0 engine has been scrapped and SE themselves outright said the 1.0 source files have actually been disposed of which the old scenes were originally rendered in. They would literally have to recreate the old 1.0 cutscenes from scratch in ARR's engine, which is simply wasted time in their eyes (there is evidence to suggest that SE at least were considering doing this back during ARR's development, but seemed to quietly drop those plans). Not to mention that a lot of the areas that 1.0's scenes took place in simply no longer exist gameplay wise in ARR, so they would have to use existing stand-in areas for them.

Having said that, we have seen some old 1.0 scenes return remade in the ARR engine though already (Raubahn's Colleseum fight in the Echo during Stormblood's patch story is an example). Unfortunately I don't see restoring all the old 1.0 scenes to the Unending Journey for 1.0 players, as nice as it would be to have them.