Because he could be persuaded on certain issues. he proved it by leaving 2,5000 troops there. Ultimately after 20 years and $2T and many lives later if the situation hadn't improved, who is supposed to believe it is a worthwhile cause? Unless you go direct to the American people and advise the world accordingly. No one had the balls.
Maybe Trump still would have completely left, it's what Americans wanted. However, even if he did, he would have been more efficient and wise about it. Really is there any way this withdrawal could have gone worse?
At lest the Old Establishment who haven't been right abut much for 30 years can say "I told you so". How about explaining to the American people why a similar situation to East/West Berlin or North/South Korea was necessary. Even with 2,5000 troops, most Americans would have understood. Would that have helped after 20 years of sacrifice? I bet it would have.
Instead, it seems the old "we support the status Quo" was the downfall, just as it will be in NOT being vocal and definitive about Taiwan, or Hong Kong, or the Winter Olympics coming up in Beijing...