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A variety of reasons, but mostly:

  1. SEO - As long as that content is on those older "original" posts on steemit.com, that is where search engines will send people, rather than my active blog on Hive.
  2. ^^^ That is literally the only value of steemit.com, and by removing my content, I am taking away some of the value of that soon-to-be fully centralized chinese website.