That's one of the reasons I don't like Notch. This game is literally perfect for WebGL. The rendering done is minimal, the CPU work done is minimal, the textures are (or could be) tiny, and the code required would be tiny.
You don't like notch, who is one of the nicest and modest people in the world, because of a programming decision he made on his game and his product? You may disagree with a business decision, but saying you dislike someone personally because of it is a bit much.
Possibly because that's what Notch was comfortable with, it's a valid choice, and he built it and made big stacks of money instead of agonizing about what language to use on the grounds of whether people would think he was cool.
But Notch didn't like seeing alternative clients, so I removed it from the game.
Also, see http://wiki.vg/Client_List