Due to high quality and return, the main buyers are banks, insurance companies and pension funds, as well as trusts, foundations, companies, etc. The largest sectors are american and japanese pension funds.
However, due to economic reasons, many banks, insurance companies and pension funds cannot be purchased directly from issuer banks of the primary market.
The reason is that if they can buy these documents directly, they will not see any reason to provide credit to the economy. Banks, insurance companies and pension funds are dependent on the secondary market, where prices are much higher and yields are very low.