I think the primary reason for the rise of increasingly heavy sites is that animations and visuals can be used to attract and "hook" your reader.

Just as fish like shiny spoons and minnow lookalikes and monkeys like shiny objects, humans like pretty pictures and flashing visualizations.

Distraction is the same principle that drives the success of TV. It is so damn easy to just sit in front of the screen and grok out, never mind the fact that the signal to noise ratio is often astonishingly low.

Quality thought and challenging content consumption is much harder than simply letting yourself admire shiny visuals. Therefore, simple websites, while they may contain excellent and meaningful content, will often not stimulate the user's interest as much as animated websites with large pictures.