If you want to understand the main difference between effective (reliably effective) closers and other relievers, the answer is simple - closers have plus plus pitches that can be reliably thrown for strikes at will. Most of them have only one or two of those bankable strike pitches, but they have them. Good relievers who struggle as closers are "effectively wild" and win with movement, deception, or change-speed games that are hard to predict.
That strategy will not work in the 9th inning - yes, Virginia the 9th inning IS different. In high leverage, last-chance scenarios, it is a demonstrable fact that big league hitters take more pitches. This leads to increased BB and K rates on the whole, but for pitchers like Arthur Rhodes, Jeff Nelson, and Carson Smith, it always has led...and always will lead...to disaster in the 9th.
Smith throws a mushy, wild fastball (which is now coming in more slowly because he's desperately trying to aim it) and a hilarious, Nintendo slider that he has no idea where it's going, but it's so huge that it constantly makes hitters look silly. If they swing enough. In the 9th inning...they don't.