Well, for one thing, starting one's own practice and becoming an employee aren't really the only two choices. Many radiologists still take the traditional partnership-track jobs, whereby they may temporarily be considered employees. But eventually they'll become full partners, who - depending on the legal structure of the practice - take on a lot of risk. To answer your question, though...
Risk is a big part, but that's not really the whole story. Running a private practice is a lot of work and takes a fair amount of business acumen. Starting one takes even more of both, and those are things that many physicians - not just radiologists - are becoming increasingly less interested in. It's also a question of market saturation, meaning there really aren't any markets out there with a radiology demand but no supply. There are probably some exceptions to that, as in maybe a hospital in an oil-boom town in North Dakota is expanding and would love to have a small private practice take on its new radiology contract. But, exceptions notwithstanding, starting a new practice means pushing other radiologists out, so it's a zero-sum game for the profession. And frankly, it's one of the most loathsome aspects of teleradiology firms. To wit, they undercut and push out the local group, and all too often quality suffers.