I think it's mainly because of Paul Graham. He's a visionary and he's excellent at spotting awesome teams (even if their ideas suck). Nobody would have thought twice about Airbnb, but Paul saw something that other investors didn't see. He noticed how effectively the team worked together. He also knew they would pivot if they needed to. Having that type of discernment has allowed YC to find & fund all these amazing startups. His hunch with the Airbnb guys paid off considering the company is valued at over a billion now!