The one in the refit enterprise was better because it looked like something was actually swirling inside. I don't know why they couldn't just duplicate that for TNG.
They probably could have - heck, they probably could've kept the existing core. I suspect that they didn't want it to look exactly
the same, even if they were using the same framework for the set. Much like how the corridors at least got new carpeting and paint.
The new one did have at least one advantage: they could make it go into "more power" mode and move the blinkies faster. Added drama.