The trend today is that more and more people and surfing the internet on their smart phones. There is a fundamental design difference between a regular website and a mobile website.
On a mobile phone, there is a very limited amount of screen space. If you have ever had to side scroll to view something on a website, you realize how much of an inconvenience it is. This is magnified on a mobile device. Even with the ability to turn the device lengthwise, the resolution is still extremely small