Sometimes, your request for administratorship/bureaucratship may not succeed. If your request is declined, don't take it in a negative fashion, it just means that the community doesn't think you're ready yet. For most users, it takes quite some time for their administrator/bureaucrat rights requests to be approved and you can apply again in the future once you've acknowledged the raised concerns.
The Sims Wiki has a group of users known as administrators and bureaucrats, who have tools that enable them to block a user, delete a page, protect a page amongst other things. These rights can only be obtained from a Request for administratorship (often abbreviated RfA) or a Request for bureaucratship (or RfB). Selection of these positions is based on community consensus, and typically only the most active and experienced editors on the wiki are able to attain these ranks. Often, a user may apply for a promotion to administrator or bureaucrat, but have their application/nomination opposed. Most of the time, this is due to the applicant's inexperience, or due to certain circumstances which can change over time. It is important to remember that a rejected RfA or RfB does not mean that future requests will also be rejected - sometimes, the answer to a request is "not now," but that answer may change in the future.
You haven't been here for very long - an application for a user right shouldn't be your first edit.
You haven't made very many contributions, whether it's constructive edits or overall edits.
You have a recent history of disruptive behavior (wiki or IRC) or you have just been released from a block.
You have recently just become a rollbacker/administrator and have yet to become accustomed to your new tools.
Your usage of your rollback/administrator rights has been questionable by members of the community.
You may need to work on a specific aspect of yourself as a user, whether it's down to usage of administration tools, activity, attitude etc.
Usually when a request is declined/opposed, users tend to assume good faith with the belief that the user asking/nominated for the rights has their heart in the right place and is trustworthy enough. Not all of the above reasons may explain why you were declined/opposed with a rights request - they are just common reasons found both here and elsewhere. More often than not, a user will tell you truthfully why you were opposed/declined.
Things to note if your request was declined
You are welcome to reapply later once you've addressed the concerns made by the community which ultimately lead to your previous request being declined.
If you were declined for disruptive behavior and you have worked to improve your conduct then users will notice that and forget or forgive what happened in the past.
Comments made on your request aren't personal but they are rather referred to as "constructive criticism", which tells you why you can't have those rights today but maybe in the future.
Even the greatest editors have had to wait up to and sometimes over a year for admin/bureaucrat rights to be granted to the user after working constructively to improve the wiki. As they say, "Be Patient. Good things come to those who wait".
What you shouldn't do if you were declined for user rights
Don't worry about it too much. It's not the end of the world - there's always next time.
Don't leave The Sims Wiki. If you hated it here, you wouldn't have considered asking/accepting a nomination for rollback, let alone administratorship or bureaucratship.
Don't refuse assistance from fellow editors. The community often help each other - even bureaucrats need help now and then.
Don't reapply immediately. If you're still interested in admin/bureaucrat rights then it's best to acknowledge the constructive criticism you were given and try again later. Reapplying immediately will just make others more hesitant to support you in the future.