There's always some sort of hype thing going on. If you made a chart of all the industry hype going back at least the 1990s there's always something. If I were older I could probably push that date back further, I'm just sticking with what I know.

I think there were a lot of factors. One is that while it's not really true there were no interchange formats, they were all either bad, locked away behind huge paywalls, or unknown to the common programmer. It is hard to conceptualize how the industry worked before the Internet meant ten seconds could lead you to anything you like.

Another is that XML was the evolution of some pre-existing standards and the people who were behind those standards, which includes some companies who made lots of money on those standards, were very excited about an evolution and saw an influence and profit opportunity in hyping it up.

Money in general feeds a lot of the hype cycles. Even before we account for people deliberately pumping them, something that becomes slightly popular attracts money, which attracts marketing, which attracts further popularity. You can see that right now in the AI craze. (Note that being a hype cycle doesn't have to mean it's all hot air, there can be something useful in the middle of it.) Then we account for people pumping it and marketing it on purpose and the hype cycles only get bigger.

XML also got a sort of halo effect from the sudden popularity of HTML and the web. It claimed to be the natural next evolution of the web technologies, even though that never really manifested. (It didn't completely fail, either, like with SVG being XML, but it didn't take the world over either.)

It's a lot of things. It's even that it is a good solution to certain problems that didn't have a good interoperable solution at the time; even today a significant portion of XML's bad reputation isn't that it was actually a bad technology but that it was applied to tasks it very much shouldn't have been. (See "What XML is good at": https://news.ycombinator.com/item?id=11446984 )

But if I had to pick the one thing that HN might be underestimating, it was the amount of money being poured into marketing in order to drive hype cycles so that you had to pick up consulting services from Sun or IBM in order to stay current. These marketing teams bought editorials, magazine covers, would create entire conferences from whole cloth for these things, and a lot of companies would dutifully send their employees off to them in order to stay "current".

There's a lot of money to be made in these hype cycles in consulting companies and companies selling shovels. A more recent one is the whole "data lake" thing; if you've not been part of it you may not realize how much money there is in selling to organizations the idea that if you just shove lots and lots of data into a pile and let Data Experts poke through it they will inevitably discover amazing facts about your company that you had no idea about, like, "if customers have been using your product a lot they're likely to continue to" and "if customers are toning down their usage of your product, they're probably about to leave". (I sarcasm here a bit, but it's just a myth that lots of data inevitably has amazing and subtle things to learn about it.) It isn't that hard for the companies making this money to pour just a bit of that money they're making into generalized fanning of the hype cycle du jour... and rolling back around to the topic, XML got a lot of it.

The most amusing thing about these hype cycles is that they generally precede people understanding the tech and generally the maturity of the tech. Java is #1 today because Sun poured immense amounts of money into a hype cycle... yet in my highly opinionated opinion, Java wasn't actually good for much of anything until after the main cycle. XML was the same thing... people generally thought of it then just as many do now, as just some angle brackets and some vague rules, but it's actually a particular thing with particular uses (https://news.ycombinator.com/item?id=11446984) and particular tendencies, which, looking back, were not well understood by much of anybody until after the hype cycle died down. The XML-based technologies that date from the height of the hype cycle are not just verbose and unwieldy, they're also poor users of XML! One of the clearest examples is Atom versus RSS; while I'm actually generally on team RSS Is Good Enough, Atom comes from an era where people generally understood XML and it correctly specifies things and uses the features correctly, whereas RSS generally dates from the "XML is angle brackets and some attributes" era of XML, and as they solve essentially the same problem provide a really clear contrast.