UA essentially got out of the cargo business when the (profitable) pre-merger CO cargo operation was outsourced to match the UA side of the house. At a time when DL and AA are actively seeking to grow their respective cargo operations, UA considers it non-core and, like everything else at this company, is a target of aggressive cost cutting.
They screwed up the entire business when they were merged with CO. But with the down gauging of international routes, and domestic routes, its a lot harder to haul a load of cargo on a 737 or worse, a 145, than a 757.
One area where United was particularly strong was the Hawaii-mainland, where UA was able to haul gobs of cargo back and forth. Obviously, its harder to do that when they replaced 767s and 777's with 737s, which lately have had to drop even some self loading freight to make it.