Pagination is one of my biggest pet peeves, and Avoid the Pains of Pagination does a good job of explaining the primary reason:

Users have better experiences with scrolling than clicking. The mouse wheels, touchpads and touchscreens of today make scrolling faster and easier than clicking. To get to the next page in a pagination, the user has to find the link target, hover the mouse over it, click it and wait for the new page to load. That’s a lot of work when you break it all down. Imagine the user having to do that over and over because of short pages. To scroll, all the user has to do is flick their finger on the mouse wheel, touchpad or touchscreen. There is no wait time and new content comes to them with minimal effort. Stop making your pages so short and allow users to scroll for more content instead of clicking.