The reason why Erik Ten Hag refused to bench Onana is that Erik Ten Hag made David De Gea leave Manchester United and thought that Onana would perform better than David De Gea, but since Onana has been performing poorly since he arrived at Manchester United, Erik Ten Hag will not want people to criticize him for letting David De Gea leave, so Erik Ten Hag will keep on starting Onana in every Manchester United match.
This reasons isn't logical enough and is not making a lot of sense to me because he's putting both his job and the name of the team on the line just because of his ego which is very wrong and shouldn't be tolerated by the management of Manchester united but then obviously the management is yet to take a decision about it.
De gea was a very much better keeper for Manchester united who would have literally helped them go far in either of the leagues they have been participating so far, he's skilled so well that he makes sure to maintain a good number of clean sheets before the end of the season but with onana he's yet to maintain UpTo 5 clean sheets this season rather he was the keeper who concede the most at the UCL group stage, such a poor result. Ten hag has to drop his ego and start putting things right els he may loose his job soon because even the fans are no longer happy with him.