Why? B/c there are no girls in silicon valley. If you are in your 20s, or early 30s, you are probably wasting your best dating years away.
I like to work hard/play hard and NYC seems the perfect place to do it. In SV, you work hard/get more bored. You are surrounded by boring suburbs. SF is fun for a while, but good luck finding dateable girls. (If you are into hipster chicks, it is perfect). Eventually work hard without corresponding fun becomes tedious, and people burn off, and move on.
NYC seems to have a better balance. When people leave NYC, (or Manhattan), they seem to settle for the cheaper boroughs, or New Jersey / RI. When people leave SF/SV, they just move away.
Fred Wilson should start emphasizing this more in his posts. He doesn't realize how bad the 'dating' situation is here, and just be in a city where you meet lots of well educated girls, is a huge perk for many single 20s-30something engineers.
Ps. I have been talking with some friends also, that are in the same boat. Our plan is to actually have at least an office there. Seriously. I guess it is just like rock bands, where people join them "to get the chicks", so will be my next startup. "To get the chicks". Life is too short. :D