For the same reason that many pub landlords discourage discussion of those topics on their premises: They're the ones which people feel most strongly about.
Further, questions on those topics generally invite opinions (of which there could be an infinite number) whereas questions such as "which airlines fly to . . . " solicit facts (where there might only be one answer).
If someone could come up with a question that manages to combine "See How Far This Goes..." with a few horror stories about immigration, Islam and and few quotations from the Jehovahs witnesses to boot, then Ab Editor may be looking at having to buy some extra hard disk space ...