Fewer American investors are benefiting from this stock boom than from past runs-ups. That's because public companies are bigger but there are less of them, with fewer middle-class investors.

The Wall Street Journal has a smart look at the big decline in companies going into the publicly traded stock market. The numbers show a 33 percent drop in the past 20 years, to 6,000 companies as of June of last year.

Why it's happening:

• Big investment funds are pouring money into private companies
• A record-number of companies are going private through mergers, or getting bought
• The scrutiny and pressure of being a public company can be a drag for CEOs