[jcbrand]

As a bank, you can't easily liquidate them without that showing up as a huge loss, thereby affecting your reserve requirements and potentially making you insolvent.

If you don't have to sell them and can keep them to maturity, they don't appear as a loss in your accounting.

It's the difference between mark-to-market versus held-to-maturity accounting. If you have to sell, you are forced to mark-to-market. If you don't have to sell, you don't have to account for losses due to bond prices falling in the market.