I know very little about that world, but I heard Dodd-Frank made going public less appealing. The first google hit for "IPO over time"[1] shows a drop after the dot-com bubble around 2000 and not much around when Dodd-Frank got passed--but it seems surprisingly flat. Money has been really cheap since the recovery. This, plus the amount of VC money, has likely made the threshold for IPO much higher.

[1] https://www.statista.com/statistics/270290/number-of-ipos-in...