If a design flaw is found for a particular browser, its far easier to find and fix the problem in the HTML source of a simple design, compared to a complex one.
Browser compatibility
All of the browsers out there, from Mozilla to Internet Explorer will display a layout differently. Distortion of an original design can be kept to a minimum by removing the amount of complex tables, cells, formatting tags, images, etc.
Bandwidth efficient
Images, flash, and tags wasted on creating a complex design all require bandwidth. Bandwidth costs money. With a bandwidth efficient, simple layout a website can have more visitors while saving on costs of bandwidth in the long run.
Page load time
Some visitors will be using a 56k or slower modem. A 12kb page loading for a visitor using high speed internet will take less than a second. That same page will take four seconds to load with a 56k modem.
