There are a multitude of reasons. The main thing is to speak to your listing broker. He is very much akin to the marketing manager for your property. The main question that will answer all the others would be to ask what is the marketing plan for your home. It is important to have a valid set of expectations on the manner in which your property is or is going to promoted.
Many Realtors still are not computer savvy know nothing about these real estate sites. Disadvantage to any property owner.
Lynn911 Dallas Realtor & Consultant, Loan Officer, Credit Repair Advisor
The Michael Group - Dallas Business Journal Top Ranked Realtors
If it has been on the MLS for a while and still not showing up then ask your listing agent to get it up there for you. When I get a listing I market it on Zilow, Trulia, Realtor.com, craigslist and all of the other major real estate sites, but it actually takes some work by the listing agent to get this accomplished. Good luck with your sale.