I think what matters for 3D maps is that Singapore is new. Pretty much everything was built in the last few decades. Anything older than that is now treasured heritage, and would have been studied to within an inch of its life. And the buildings are big; no doubt some architecture firm still has the Autocad files for half of them.

This would be harder in a city where lots of things date back to the 19th Century, and architects were rarely involved in building them.