Because it runs in a sandboxed browser environment, it cannot access your file system for mods (like Forge) but can use localStorage for saving settings and worlds.

"Pure HTML5," Jake grinned, adjusting his glasses. "Single-player, right in the browser. No Java installation needed. They can’t block it because to the firewall, it just looks like I’m loading a heavy webpage."

This is the most contested aspect of Eaglercraft.