Can anyone out there help me understand why there aren't more educational physics "applets" written in HTML5/JavaScript?
A decade ago, dozens--possibly hundreds--of us were writing Java applets. But support for this environment is disappearing very quickly, and it never did work on mobile devices.
The only viable replacement for Java applets is JavaScript and HTML5, especially the canvas element which allows you to draw anything you want from JavaScript. I formerly assumed that JavaScript would be too slow for computationally intensive simulations, but this is no longer true--especially in Chrome. I've whipped up a few proof-of-concept examples which are linked from http://physics.weber.edu/schroeder/software/.
If you google for "html5 javascript physics" or something similar, you find almost nothing but pages about game development. When I google "html5 javascript physics site:.edu", one of my own pages is the third hit(!)--and almost none of the others have anything to do with teaching physics.
I've recently learned of Bruce Sherwood's ultra-cool GlowScript project: http://www.glowscript.org/. This adds another software layer between you and your web page, and browser compatibility is still pretty limited, but it's extremely impressive for what it does.