From the talk by the Caltech professor leading LIGO, the theory was good on paper but the engineering problems were very tough. The platform to make measurement has to be absolutely still. But the Earth always has some movements due to seismic activities. For years they just couldn't get the passive vibration isolation tech stable enough for the measurement to work. The project was almost canceled. Finally they went back to the drawing board and partnered with some companies to build new active vibration isolation technologies. It took years to get it working.

Their active vibration isolation technology is insanely good. It basically detects tiny seismic movement of the Earth far away and actively compensates the stable platform. This is one instance of scientific project spawning off new technologies, which will have many other uses in the future.