[fiddlerwoaroof]

Part of the problem here is that desktop development frameworks included keyboard shortcuts essentially for free: Mac associates them with menu items, emacs/vim's key -> command -> action design give an obvious place to implement shortcuts; Delphi and similar programs had similar places to automatically slot in keyboard shortcuts. Web frameworks focus on visuals and mouse interactivity and keyboard control is almost an afterthought (aside from some minimal attention to tab-order because of accessibility).

[mook]

Another part of the problem is that the contents of the browser is mostly untrusted, and the browser itself already has a giant pile of shortcut keys that would conflict.