This game isn't exactly simple. There's an entire meta language called WML (Wesnoth Markup Language), support for a powerful AI and Lua scripting. There's also very intricate multiplayer code that I don't think in any way could be trivial with JS.

Yes, Wesnoth is hamstrung a little by its tech stack. The dev team very much wants to move to SDL 2 and support OpenGL in the future.

How will they do that? With more and interested developers. Hence, the call to action.