Convoy effectively got into the factoring business and extended short term (30/60/90 days) loans to the trucking companies. Now, they don't hold these loans on their balance sheet but rather package them and sell them to lenders as asset-backed lending portfolio.
The thing with these is that typically the originator (in this case Convoy) will need to take the first tranche of losses. (This is where the $240M debt facility came in). As the freight market deteriorated, Convoy was effectively margin called by the lenders and could not come up with the money.
Any acquirer would then have to take up this debt. Even though Convoy may have a valuable asset, UPS is not in the business of managing a debt portfolio. Any acquirer would be dissuade by this baggage.