Why the advantages of this type of investments in the Secondary Market (accessible to big investors, governments, companies and banks) are unknown by the majority of the general public?

This is due to the Glass-Steagall Act which was issued in the year 1930 by the European countries that now form the European Union and the United States through its Central Bank.  
The Glass- Steagall Act expressly prohibits direct contact between a bank and a private investor. Instead, communications should be established through the figure of a Commitment Holder or Bank Trader, until 2004 there were 11 worldwide but now there are less than ten. 

However, the Internet provides information in the existence of these types of financial has done that many people know about the existence of this type of financial programs, although some can be misleading with wrong and even fraudulent information.  

To enter the Secondary Market, the Glass-Steagall Act orders that an investor must be invited for an institution (through a Bank Trader) after verifying the documentation of the possible client.  
The Glass-Steagall Act does not allow banks to contact the investors directly and the investor cannot contact the bank directly. These contacts are exclusively taken on board by the Bank Trader, assisted by a consulting team, which is responsible for requiring and verifying the needed documentation.