Renovating commercial office space for residential use would be very expensive. Just re-plumbing alone would not be cheep. Probably cheaper than building a brand new building, but I don't know the numbers on that.
I've read that in cases where it is not a skyscraper it is more expensive than knock-down and rebuild.
And unless it's a really cool old industrial building, you end up with crappy residential buildings that aren't tremendously desired unless the location is really, really good.