Because, to the apparent dread of many on this board, Raleigh is and always will be more than just a downtown (which still is barely a downtown). The West Raleigh complex of 5000 acre Umstead State Park/state fairgrounds/165 acre Museum Art Park/Raulston Arboretum etc. etc. makes this area a destination for the entire Triangle, and many points beyond. There is zero advantage to NC State to move its world-class arboretum, particulary now that it has such a lovely education center. If only the art museum had been built downtown, as many now lament, there would never have been an artpark, which seems to be very popular right where it is.
I think I recall that somewhere who knows how many pages back in this thread someone had the idea of moving the Art Museum to Dix. Instead of trying to steal the best of Raleigh and relocating it where it doesn't fit, how about coming up with something original for Dix?
I concede that the arena should have been built on Centennial Campus, but there's no moving that now.
Jones123 above gets it - there are neighborhoods in Brier Creek that are more dense, more walkable, and more multi-use than Boylan Heights. The Dix property is beautiful just the way it is. If it's not good enough for some, with a little original thought and a good bit of restraint it could be transformed into one of the world's greatest parks. Try to cram a botanic garden and a giant ferris wheel and an ice rink and an aquarium and whatever else anyone in the city can dream up, and it will become so much less than the sum of its parts.