Each search engine, directory, etc. has it’s own rules and requirements for accepting submissions so there is no all-purpose answer to this one.
Here are some possible reasons why your site wasn’t submitted to one or more particular search engines:
- the engine’s requirements do not fit your site
- many do not accept free submissions at all anymore
- free submission requires a reciprocal link (a link back to the engine) and your site does not have a links page or section
- the engine requires a physical location and you do not provide that information on your site
- the submission process requires an email address from the same domain name as the URL being submitted and you are not using your domain for email