Because as literally dozens of people have already pointed out, it often wasn't actually a duplicate.

"How do I do X in Python?"

"This has already been answered in <other question>. Closing as duplicate."

And then you look up that other question and it's "How do I do X in PHP?"

Ask how to prevent XSS, and someone will flag it as a duplicate of a question asking how to prevent SQL Injection. Ask how to encode something in Base64, someone will flag it as a duplicate of a question about how to encrypt something with AES.

And the worst part, is that these moron moderators pat themselves on the back for keeping SO clean.