One of the comments state "I don't live in the US", which is my reason too, but let me elaborate a little.

Simply put, while YC seems great, it's just too expensive for me and my co-founders. There is three of us, and if only two of us engage actively in the process we would have to spend over $1200 just for getting to the interview (based on a quick Expedia search, and not including any car rentals, visa fees and the like); and if we get accepted, just getting us over there would spend at least 10% of the supposed YC investment.

It would be great if YC would open a European counterpart, but that don't seem likely as they have closed off even the East Coast one.