Because they have very little to offer. Without it, your downloads slow to a crawl. But with ample bandwidth on our side, your downloads can zip at scorching rates, even over a dial-up access line and 28.8 modem. It's really simple. Consider a small town's water tower.
Big pipes mean lots of bandwidth, and the ability to serve large numbers of downloads at once.
Also important is bandwidth served from various points on the internet. If you're in California, for example, downloads from a New York site will usually be slower than downloads from a Los Angeles site, owing to the greater geographical distance. More than just about any other commercial site on the net, and more than all but the largest Internet Service Providers. So don't throw money away into a low-bandwidth adult web site where the downloads remind you of a leaky faucet.