Media outlets publish their stories based on which ones will be the most popular. The big clubs have millions of fans interested in what's going on, of course they get more coverage. An average United/Liverpool/arsenal story will clock up hundreds of thousands of reads, whereas saints stories maybe 10-20k on a good day!
This is exactly the reason why the BBC should cover every other club in a fair and balanced way - they are not funded commercially so they have no excuse. They pride themselves as balanced, but all too often use their power to promote popularity rather than discussion.

