Sarbanes Oxley passed in 2002. IPOs increased from 2002- 2004. Source: https://www.quandl.com/data/RITTER/US_IPO_STATS-Historical-U...
I suspect that Sarbanes-Oxley did suppress IPOs but if you can't think of some other reasons for fewer IPOs. I will offer some:
- Much more capital available in private markets from hedge funds, sovereign funds, etc. Think of how much money Uber can raise today as a private company. That didn't used to be possible.
- Tech companies have really resisted offering dividends because they are believed to be tax inefficient and a sign that those companies can't grow and invest. So instead they have huge amounts of cash on their balance sheet that they need to invest.