It has not. The debt was conditional on the acquisition of Twitter by Musk, so Morgan Stanley could not sell it before the acquisition was completed. But the debt was necessary for the acquisition, so the acquisition could not be completed without the debt money. Therefore, they had to happen at the same time, with M&S fronting the money.

Of course the terms of the debt, and the price of the acquisition, had to be both defined for everything to move forward, so they were finalized months ago.

When Musk signed a contract with Morgan Stanley, money was cheap, so the debt has very low interest. When the debt has actually been issued, a few days ago, money had become more expensive; Morgan Stanley won't find anyone to buy it as-is. To sell it they will have to discount it quite deeply, losing a bunch of money; otherwise they'll be stuck with it.