One reason why you could have been rejected by YC or others

It's simple. They may have funded a start-up that is doing exactly what you are doing in the previous class. After receiving rejections from a lot of incubator programs, one thing i have realized is that there are others who got started doing exactly what we do (or close enough) before us. That said, we have just enough chance of success, but we may have to do it by ourselves. Incubators don't like their children squaring off, and they rather put more effort into an existing baby than another new one. So, if you think your idea was world changing and got rejected, give this possibility a thought- there may be another company doing what you do and you just haven't heard about them :)