Fashion changes, and what used to be considered 'good-looking' few years ago is not necessarily perceived the same nowadays. There was a time when animated gifs and tacky music were the hottest things in web design, but today all of these are far from looking advanced. Internet technology has changed dramatically in the past decade leaving behind all those pioneer websites. You can distinguish between an outdated website and a modern website in first sight by looking at the range of colors and design layout, or the lack thereof.
For many online businesses who don't have a physical location, and even for those who do, appearance is the only way to be introduced to the public and to convey a message to potential customers. Naturally, a modern website is more likely to attract visitors and keep them coming back for more.