From what I've heard, In YC's case:
+ Paul was a successful technical founder with the money and reputation. He could rally the troops with "suits and MBAs don't matter, hackers rule the world" rhetoric.
+ Paul had a giant megaphone with his blog (not initially done to market YC)
+ He had a partner who was good at reading and sizing up people, crucial in picking the first batches whose success was crucial to the whole thing working.
+ The cost of doing a start-up had fallen dramatically
+ and so on. I am sure there a few others.
Similarly, everyone has tried to replicate the Kenya's mPesa, the world's most successful mobile money system. No one has succeeded because there hasn't been the same confluence of factors that led to mpesa's success: