I'm a great believer in engaging in some relevant research before embarking on a new project. So before I wrote the first draft of my first mystery book, Operatory of Death, I did some reading and thinking about what the elements are that constitute a good mystery story. An interesting plot and well developed characters are important features, but this applies to any book. In many mystery books, whether graphic or cozy, it is necessary that the perpetrator, the 'guilty party' remain unknown to the reader, at least until near the end of the book. I know there are exceptions to this depending upon the storyline.
I have read reviews of mysteries wherein the chief complaint was that 'I knew who it was by the second page . . .' or 'it was quickly obvious to me who the murderer was . . .' Most readers, including me, like to be kept on tenterhooks so there needs to exist several possibilities. Too many, though, and it can become ridiculous although I must make an exception for Murder on the Orient Express. I won't give the ending away in the unlikely event that someone reading this hasn't read Agatha Christie's classic.
Not only must there be several possible perpetrators, but often readers like to be surprised, at least somewhat initially , and then they want to immediately be able to trace back the clues that now, once the facts are known, make it all too obvious as to the identity of the guilty party. For writers, this take some effort and planning in the writing process and the time that involves but no added expense. With some television shows I've seen, no doubt on tight budgets, there is only one guest actor and unless a regular is going to be written out of the series, it soon becomes obvious who 'did it.'