In most cases the decision to form a hedge fund is based on economics. Hedge fund managers can typically earn more than money managers because hedge funds can usually raise more capital than "unregistered" money managers. This is because unregistered money managers are limited in the number of clients they can have. Under current federal and state law money managers are considered to be "investment advisers." Federal law allows unregistered investment advisers up to 5 clients in a 12 month period--although some states allow more.
In order to obtain more clients, the money manager/investment adviser must become registered by passing certain exams, filing disclosure documents (Form ADV) and complying with very strict state laws.
However, each hedge fund is only 1 client--even though the hedge fund can have numerous investors. Therefore, a hedge fund manager can raise significantly more capital without becoming registered with the SEC or state securities divisions.
Let me know if have additional questions. My contact information is on Elite Trader.