YC is looking at this from the VC/startup perspective. "Does this group of people with this company seem like they can make a billion dollar business in the next ~10 years?" They want founders that target a huge (or growing / potentially huge) market with stars in their eyes and some insight as to how to own it.
If you're content with a small business and growing slowly then you are not a good fit.
You might have a great business in the end, but still not be a fit for the model. Like others have said, YC partners miss big hits. They're not perfect. Keep going and if you have a relentless focus on month over month growth and keep your churn down then you have a chance to get VC-style growth capital. But, if you just want to bootstrap a profitable business you'll be better off doing that on your own and getting bank loans or other risk-adverse capital.