What you need to remember is that Taris is an ecumenopolis - an entire planet covered in cityscape. Very few full planets, even in Star Wars, boast the same - Coruscant, Corellia, Alsakan and Anaxes are some of the few. Even Nar Shaddaa is merely a moon, far smaller than Taris.
The resources required to recolonize and reconstitute Taris are a microfraction of the resources required to create an entire ecumenopolis even on an already mostly urbanized planet. The original infrastructure is, according to several NPCs, "Surprisingly intact" as well.
It seems to me that it is like the world trade center's, it would cost more to clean up that rubble pile, then it would cost to build new buildings.
not the same scale but sometimes its easier to clean a slate and start again, the only reason Taris is being rebuilt is because of the symbol it provides the republic, and with a Sith empire on my arse i would probably be more worried about how many ships i can built this week, not how many repairs i can do on Taris.