Why was Dynamic HTML a bust and "AJAX" a hit?

One of the eternal mysteries of the software industry for me is how formerly ignored or scorned ideas can become buzzwords du jour almost overnight.  XML  -- which has a data model that is quite close to the scorned LISP S-Expressions and uses the "discredited" hierarchical rather than relational approach -- is one big example.  A smaller but more contemporary example is what Microsoft and Netscape called "Dynamic HTML" back in 1997 or so, but has become resurrected as AJAX - "Asynchronous Javascript and XML".

There have been a several of interesting pieces recently on why this idea went from hype to passe to waycoolbuzzwordcompliant in just over 5 years.  I'll single out a couple from people who were present at the creation of XmlHttpRequest and have seen it rise, fall, and rise again on the popularity charts: Adam Bosworth helped drive this at MS in the late '90s, and now observes:

the physics didn't work in 1997. A lot of Ajax applications have a lot of script (often 10 or 20,000 lines) and without broadband, the download of this can be extremely painful. With broadband and standard tricks for compressing the script, it is a breeze. Even if you could download this much script in 1997, it ran too slowly. Javascript wasn't fast enough to respond in real time to user actions let alone to fetch some related data over HTTP. But Moore's law has come to its rescue and what was sluggish in 1997 is often lightning quick today.

Finally, in 1997 or even in 1999 there wasn't a practical way to write these applications to run on all browsers. Today, with work, this is doable.

Kurt Cagle  wrote an article about XmlHttpRequest in 2000 and recently discovered " I'm so bleeding edge I'm surprised I haven't died from blood loss" now that AJAX has more buzz than a hyped-up hornet.  He attributes the buzz to the fact that: 'Opera, Safari and Mozilla Firefox all adopted the XMLHttpRequest object as an inbuilt component with KDE's Konqueror recently joining the ranks. This was a critical turning point, as it meant that at least desktop clients now have the capability of communicating with the server "out-of-band".' 

I guess the observation that everything old becomes new again in the world of fashion is hardly novel, but those who live on the bleeding edge need to recall it now and then.