The larger the site gets, the more time intensive it is to manage, and the more difficult it gets to provide the new information that people need. The site evolved out of my own work as an environmental consultant. The intention was never really to reach the size that it has - the growth has been spontaneous. For example, I get a lot of requests for email for documents. I often put these documents on the website as a means of making them available to the person who requested them.
Opportunism also plays a major part.
As is discussed in the 'Usage' section below, thousands of pounds of government publications are made available through the website every month. There are also a wide variety of other reports and briefings on the site produced specifically for local campaigners. Because it's a useful service to communities not only in the UK, but across the world. By making a donation you are supporting the work which supports those who make use of the site. Because you can.