Conditions for an IPO are rarely perfect - you need to have the right internal situation, the right market conditions etc. etc. - so if the company is thinking of doing an IPO, and they can ... well then they're going to lean towards doing it.
VC's can cash out if they want and pass the buck.
That said, if they only have some major 'one time cost' dragging down earnings, and earnings will be spectacularly better next year ... it might be worth the wait. But even then - if the IPO suffers a little, but then rockstar numbers come out later, the stock will go up. So VC's that want to hold on have the option to do that.