Niche is by far the hardest to get into because you usually have to have specific skills and real knowledge of the field. Like say medical photography which I do have some experience with. First you probably need to have worked at a hospital as a staff photographer for a while and that requires an education in photography. For the job I had its was at least a B/A in photography and 5 years professional experience and a smoke'n portfolio. The reason everyone isn't doing it is because its hard work and take years to just get started. 4 years of college for one. Weddings, yeah, thats tough. Just put your name on Craigs list.
I think you are confusing niche photography with your specific experience but the fact remains that niche photography is by far the easiest of all to get into and gain national recognition. You example of being a medical photographer takes a few years for school and OJT....what do you think it takes to be the best wedding photographer in the country, prob 50years of experience and 10K-15K weddings, compare that your your undergrad in radiology!!!
Sure if you want your niche to be photographing the international space station you must first become and astronaut! But thats just one example...If you want your niche to be corn flakes all you need is 5bucks, a camera and a grocery store! With a few thousand photos on a website all of cornflakes you could make a legitimate claim that you are the number 1 corn flake photographer in the world!
Same model holds true for the internet business...Succesful startups in intenet companies are usually the only one in their specific niche and thats what makes them popular!




