Answer by Brett Fox:
The bar for going public has significantly increased since Sarbanes-Oxley went into effect in 2002. For companies with a market cap/value of ~$300M or less, this is a real issue. Having said that, the JOBS act, passed in 2012, has made it easier for small cap companies to go public. It will be interesting to see how this plays out.
Another reason is keeping your company private longer has some big advantages. You don't have to share your results with your competitors, and you can have a long term view because you don't have to report quarterly earnings.