The problem is that if you're going to have the reader follow a particular detective around for several books, the reader will eventually want to see more sides

The problem is that if you're going to have the reader follow a particular detective around for several books, the reader will eventually want to see more sides to the character than just the professional one; treating the detective as merely a detective, solving cases that otherwise don't engage his or her wider life. begins to feel very mechanical and shallow, even--if it's not too strong a word--exploitative. Hence the "messy personal lives," stories where the criminal kidnaps the detective's spouse, etc. Nothing says the detective has to be an embittered alcoholic or some other cliché, but I think there's good reasons behind the trend.