[grzm]

Let's take a step back. How many news organizations are well-known for investigative journalism? I'd say the number is relatively small, so you're already working with a small set. Journalism is also a function of the individual editors and journalists who are actually doing the work. What interests them? What are they good at? Looking at the situation from just these, it's not all that surprising that a single news organization tends to be responsible for a majority—if not all—of the investigations with respect to a particular topic without the necessity of competition funding.