This is not least because sovereigns, as debtors, are afforded preferential treatment in financial market regulation. Banks do not have to back sovereign bonds with capital, as is required for any other loan. And since capital is tight, this privileged treatment creates an incentive for banks to purchase excessive amounts of sovereign bonds.
This is especially the case since the large exposure limit, according to which loans to an individual borrower may not exceed 25% of liable capital, does not apply in the case of loans to the government.