Samsung reviews portfolios based on the following criteria: creativity, originality, and theme development ability. If you were rejected, please apply again with your updated portfolio, which you have modified considering the Theme application criteria.
You can also refer to the Theme_Design_Submission_Template.zip, which you can also download here: Get a Theme Service Partnership Account. You may not be able to receive a reply for additional inquiries.
For more information about how to apply to become a Theme developer and reasons why you might be rejected, see Get a Theme Service Partnership Account.