1. You aren't sure what you want to say.
If your story is still rather vague in your mind's eye, then it may not be time yet to sit down and write. I'm not saying you need a super-duper, overly detailed outline, but you should know where you are headed, what the goal or destination is. For example, if you are writing a mystery, you should be able to say it was Miss Scarlet in the Conservatory, with the Knife... and her motive was revenge. Otherwise (in my opinion) you'll be flailing about.
2. You haven't done the research.
You need to know your setting, the laws, the weapons, etc. Yes, your mystery novel is fiction, but readers will only go so far with their suspension of disbelief.
3. You can't take criticism.
If you get a negative review on Amazon or Goodreads (or elsewhere), will it ruin your day? Can you take a punch? Not everyone will like your writing, some people will think your book sucks. In the anonymity of the Internet, people can say anything, and you may see comments that are hurtful. Can you take it?
4. You don't know the genre.
This applies to any genre, not just mystery. You should know the conventions, learn who the popular authors are in that genre. Think about what they are doing and why they are popular. Think about what's missing from the genre.
5. You aren't willing to spend time marketing.
This should probably have been listed as number one. Writing your book will probably take far less time than promoting and marketing it. You will need to spend several hours a week doing online support for your book so that you can build a base of readers. This is true whether you are traditionally published or indie published. Be prepared to use Twitter, blogging, GoodReads, chat rooms/forums and more. Also, you may also have to spend time doing store signings, library events, book club discussions. These things may be fun and you may indeed enjoy doing (some of) them, but there are only so many hours in a day. Can you realistically devote time to marketing your book? Will you be trying to promote your book after working at a full time job or looking after your kids?
6. Bonus reason! You're in it for the money.