Find a description of Hollywood communism hunts, such as at http://www.moderntimes.com/palace/blacklist.htm. (In order for this website to display properly, you must copy and paste this link in your browser.) This essay describes the 1947 activities of the House Un-American Activities Committee HUAC). Combining the text with your research, why did the committee focus its attention on the "Hollywood Ten"?
After some serious digging, I think I've found some good sources to help you formulate a professional answer to this very interesting question. I have mixed these sources with some of my own speculation below.
Why these ten?
Of the original ten, NONE were popular actors of the time. Imagine accusing Bogey or John Wayne of being un-American. The Herculean task would have been enough to divert the efforts of Reagan, McCarthy, Nixon and Thomas, (then head of HUAC, later imprisoned for misuse of funds) all of whose names have been stained with corruption. The common denominator in this answer seems to be power. Orson Welles said that the industry was about power, not money. Taking this into account, the studio-contracted actors would have very little influence over the finished product to be presented to American audiences. Therefore, HUAC would target those who would be able to insert "subversive" ideas into the films. Fear drove the engines of government to create camps, accuse innocents and execute spies. Not unlike today's "terrorist" hunts, fear brings about quick, un-thinking and inhumane treatment. Abu Ghraib and Gitmo spring to mind, not to mention the trampling of the Bill of Rights by the Patriot Act.
Okay, back to business.
Actors would be too popular to touch, and had little power, but O the power of the pen! At least eight of the ten were screenwriters, with questionable names like Maltz, Ornitz, Dmytryk and Biberman. Nothing like a Nazi reference in 1947 to smear someone's intentions. the writers were ultimately accused of inserting subversive messages in their films. However:
"Evidence of leftist images and dialog Hollywood films was extremely slim. HUAC had to resort to citing the smiling children in Song of Russia (1944) and noting that Russian workers shouted "tovarich" (comrade) as American merchant ships that had run the Nazi submarine blockade entered a Soviet port in Action in the North Atlantic (1943). ...
This solution thoroughly looks at the Hollywood Ten's blacklisting by The House Un-American Activities Committee. In trying to explain the Committee's specific action against these ten men, a hypothesis is formed drawing from sources such as film study sites, the Encyclopedia of the American Left, and academic journals. The author's speculation is added, based on knowledge of American cinema and theatre training.