This is a very common effect in our industry.
For example, I personally use haXe and deploy my client code on the Flash Player, because IMHO it is the best web enabled platform I can target. Once the C# backend is finished, I will probably check out whether Silverlight is worth using, although my personal feeling is, that it died, before it actually took off.
Being very happy with my language choice, a thing I ask myself often is: Why don't more web developers use an open-source, multi-paradigm, expressive, cross-platform language?
There's many reasons, but they're always the same. A valid one is personal preference. But often it comes down to ignorance or reluctance towards new/niche technologies.
When it comes to Flash, I had numerous arguments about why it has its place and why to use it. People mostly argue, that the whole point of Flash is to create fancy sites that load for ages and perform horribly (and spread a lot of other misinformation).
In fact, the opposite is true and apps such as Aviary Phoenix or Sliderocket and games as Koyotl and Tanki Online prove it. Flash is a mature platform to create desktop-like experience in the browser.
In the end, too many strategic decisions are made by incompetent people, who prefer to follow trends and rather trust some fancy blogger than their developers. And who really have a lot of wrong ideas in their head.
New/niche technologies will always struggle for acceptance, unless they really make a breakthrough. Ruby for example succeeded in this through Rails and the big hype around it. Flash had such a breakthrough for designers, because in the 90s people thought shrill is good and it was the first widely spread platform that allowed to implement just that.
You don't need to write your AJAX website from scratch nowadays. You can actually not have any understanding of HTML and do it anyway, in a language of your choice.
Right now, HTML5 is greatly advertised and pushed forward and many people infer the death of Flash from that. A lot of great reasons are given, why HTML5 is better than Flash. What is more likely, is that you'll have more and more resource hungry, bloated websites created with HTML5. Standard based crap is no better than 3rd party based crap.
Right now, there's a lot of things happening. The iPhone and other similar devices have created a giant market, that hasn't been there just 4 years ago. And the web standards are finally being driven forward by all the major companies in the same direction (vaguely).
Personally, I just hope that all the agitation settles within a year or two, that HTML5 stabilizes, matures and spreads until then, while Apple will hopefully take a less despotic stance and Flash Player becomes faster on mobile platforms. And that once this big step is done, people will just come back to choosing the right tool for the job, just as it were after the browser wars had come to stop. As of now, there's too much noise for people to think clearly.