Part of the reason is sitting right there in this sentence. You can't even bring yourself to call RSS "RSS", because the so-called "standard" has more forks than the devil's toolshed.

But the real reason RSS loses is that it's built on XML, and because nobody actually wants to look at XML a feed is read and written solely by computers, for whose convenience its design has been optimized. Twitter, on the other hand, is a protocol designed to be read and written directly by humans. It excludes all the ugly, verbose, oft-redundant tags and just delivers a message, which may contain an arbitrary number of #hashtags, @addresses, URLs, and blither in whatever order seems appropriate. The result feels human, instead of like the homework from a 1978 class on data processing. And it's really malleable: To invent a new way of Twittering all you have to do is type something and hope that enough of the audience can figure it out.

Not really sure why you say that. RSS was pretty successful for a while - successful enough to get built into every browser and most email clients. End user consumers never saw the XML and the publishing side was usually built into blog software so neither did the publishers. I think this is not so much about XML but rather a more generic notion of simplicity - the whole concept of Twitter is so brain dead simple that it removes nearly all friction in adopting it. It's a much more general simplicity than just the encoding.