One reason being Mr. Twee I-don't-use-two-spaces-after-a-full-stop, that it's awfully hard to tell if "Mr. Twee" is a sentence end or not, or if "stop. It" is supposed to be something else.
That is, in the typewritten manuscript, the double-space following the full stop carries meaning which the typesetting system (HN's HTML encoder, here), would have to guess at.
Or, you know, there's another way. Those of us who learnt to type properly can tip it off with our fuddly double-spaces.
That's a point that seems utterly lost on most today.
When I type, it's ASCII in vim.
When I typeset, it's \LaTeX (or more often these days, Markdown). Which takes care of the fucking fiddly bits for me.