MISUNDERSTANDING THE MARKET.
btw, we're not telling you to chase trends, but you must understand that certain types of story premise and characters, preferred viewpoints and more, change and evolve over time. For example, the typical gumshoe detective of the past has been replaced by protagonists more exotic and diverse. Terrorists and 911 stories are dead on arrival, and the thriller market overdosed on Nazis a long time ago, shortly after Allan Folsom's publisher launched THE DAY AFTER TOMORROW. Historical cozy mysteries are more popular while Miss Marple's many clones have sent countless sad writers to shelve their manuscripts and return to their wage slave existence.
Keep in mind too that providing good comparables in your query letter can prove difficult if you are not well read in your genre. Also, if you ever meet with an agent or editor and they question you about your genre, or what you've read lately, you'll fall flat if you don't appear knowledgeable on the subject of what is hot and/or upcoming.