Bloated specs with tons of historical cruft and edge cases. And a renderer needs to support it all. The problem stems from trying to use something designed to share documents with some custom styling as a universal GUI for applications instead.

It's possible to develop relatively lightweight browser engines. Refer to NetSurf for instance. However the ones I've seen are not what you'd consider fully-featured or usable for the average user's everyday browsing. The issues begin at implementing modern CSS and JavaScript support. Admittedly I've never looked too deeply at the internals, from what I understand supporting modern web standards is a maze of edge-cases, bug-for-bug compatibility issues and a forever moving target of feature parity with the major browsers.