I'd have thought the premise of the question was more along the lines of: YC's signalling value grew as a direct result of having great companies in its first few cohorts and supporting them well. And the ethos has been very much focused around building huge companies within VC fund time horizons. Yet the successes [seen so far] in those early cohorts chose to exit through private sales rather than push for the public markets. Is there a reason behind this?

Which is a valid question, even if the pool of potential candidates for IPOs by now really covers only the first seventy or so companies rather than the whole 940. The few companies that haven't privately exited or died in those first seventy or so companies does include Dropbox though...