Many questions like Can an unacceptable / illegal condition in a contract be ignored without voiding the whole contract? fail to declare the jurisdiction. Ought such failure be a close reason?
Tackling your questions in reverse order:
Ought such failure be a close reason?
A lot of law is based on universal principles. Even more is based on principles common to the legal system (common law, civil law etc.). For most general questions (which is the kind we like) an answer from general principles won't usually turn on the idiosyncracies of jurisdiction.
However, sometimes the question is specific enough (without crossing into giving legal advice) that jurisdiction does matter. If you think you have one of those, by all means, vote to close and leave a comment asking for it. Please explain what jurisdiction is and why it matters because if they asked a question that turns on it and didn't specify it they probably have no idea.
How can we require jurisdiction to be declared?
There is no technical way of doing this.
I am active on Role Playing Games and over there they have a user-enforced policy that a question is closed if a game isn't specified - either in the body or in the tags. That seems to work OK.