There are two reasons that Detective novels are shorter.
The first is pacing. A detective novel is generally focused, and tightly paced, sidelines to the plot are truncated 'red herrings', sub plots tend to distract from the main 'mystery' rather than embellish so are avoided.
The second is that once the mystery you started the story with is uncovered, the story is over. It can be extended, but everything after the mystery is uncovered is going to be epilogue to the main story.
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There are two reasons that Detective novels are shorter.
The first is pacing. A detective novel is generally focused, and tightly paced, sidelines to the plot are truncated 'red herrings', sub plots tend to distract from the main 'mystery' rather than embellish so are avoided.
The second is that once the mystery you started the story with is uncovered, the story is over. It can be extended, but everything after the mystery is uncovered is going to be epilogue to the main story.