I think it comes down to "open, free and painless", for spare-time projects these are all extremely important points. I want to spend my free time with stuff that is fun.
Let's look why the other stuff isn't as much fun:
iOS: The tools you need for development are all free, XCode is a very slick IDE to work in, and unlike VisualStudio there's no artificial distinction between a (feature-cut) free and a (pricy) professional version. So far so good. The pain starts when you want to run your code on your actual iOS device. Welcome to provisioning profile hell. First you need to hand over $99 per year for the privilege to run your own code on you own hardware, but that's the least of it. Next you need to create "provisioning profiles" on Apple's developer portal, registering each team member, device and application and set up who may do what. In the end you essentially get per-app/per-device code-signing-certificates which expire every three months. So all the iOS demos which I did 2 years ago don't work anymore unless I go through all that hell again. Nope.
Android: Android C++ development sucks, plain and simple. It's a pain in the ass to set up (it's less painful if you use nVidias ready-made installer), remote debugging a native app is so slow it's essentially useless, and you can't use the cool new stuff since most of the world is still running an Android version from the stone age. To be fair, this was all 1.5 years ago, but I have little motivation to waste further weekends in finding out whether things have improved since then ;)
Google Native Client: The main reasons why I stopped dabbling with Native Client is that it is still not opened up (only works with Chrome Web Store bundled apps), and pNaCl seems to take forever to be finished. To be fair, Native Client has very good middleware support (like FMOD or RakNet), but it doesn't look like it will ever be implemented outside of Chrome.
flascc: I played around with flascc for a weekend or two, 2 main reasons why it didn't set my heart on fire: (1) Compiling/linking is extra-ordinarily slow AND/OR uses infinite amounts of RAM. For reasonably big code bases (like Nebula3) it's unusable because my 4GB Mac simply ran out of memory. (2) since working with flascc is so damn slow I wasn't motivated to actually go on with writing a Stage3D wrapper for N3's rendering layer.
So all in all, emscripten is the most frictionless way to write and and actually publish 3D demos for me. I can host the demos wherever I want, update them without a certification or signing process getting in the way, the demos won't expire, they are automatically multi-platform and finally, there's no vendor or platform lock-in. Most of the code I'm writing is platform-agnostic C++ and will compile and run anywhere, and the host platform's "API foot print" is minimal: a subset of POSIX and OpenGL, which will also compile almost anywhere else with minimal changes.