They only say "no" to something you (think you) really need at this moment :) Say, https://jira.atlassian.com/browse/JRA-4446 "Sub-issues should be able to contain their own sub-issues" -- #6 on the list of top voted for.
Actually Stash issue's were handled badly. Some issue's were delayed for better "integration" with jira. unfortunatly these issue's were way more important.