The overall topic is the same, even in the hypothetical sequence you mention. Keep in mind that even if an article series is strictly partitioned into distinct parts, the discussion threads mostly won't be - all the different aspects will blend together, which means the threads will be more like "the same soup over and over" than "one about metallurgy, one about design, etc."

(Edit: I just noticed that strbean already made this point in the sibling comment!)

Also: usually the splitting into a series is somewhat artificial. In the worst cases, people try to make the segments be like TV episodes with cliffhangers, to push you to the next bit. That's a poor fit for HN. But even when they don't, to get the full "meal" you still have to go through all the parts. Few people do that, and the threads as a whole never do. This makes it less interesting and satisfying.

But there can be exceptions, and (ironically?) featuring an occasional exception mixes things up and so reduces repetitiveness! The trouble is that once people see one exception, they immediately expect/want others, pushing things back into a repetitive sequence and making the site less interesting again. It's a bit like telling the same joke twice in a row—the interest is all in the first telling.

Guess: there is likely some repetition in articles in a series, but there is a ton in the discussion here, and that is what HN wants to avoid. Discussion on a link that bundles together the parts of a series helps avoid excessive rehashing in the comment sections.