Don't know the answer to your question, but thanks for the post. Great material; I'm saving it for my files.
slimboyfat makes an interesting point. I think the answer to his question is that JWs are just so insignificant. Mormondom may be small like JWdom, but it is more interesting to the public because some Mormons are well-known. There are well-known politicians, athletes, entertainers, etc. who are Mormons. They have a university. They don't encourage the rank and file to simply be tradesmen or janitors. They simply are more interesting. Can you imagine a university in JW Land?
JWs are legends in their own minds. They think this new preaching method (listening, showing love, not having a rehearsed sermon) is so wonderful. Well, by far, most people couldn't care less about them and don't want to have conversations with them. They are simply viewed as oddball nuisances. I just don't think their is enough interest by the public to move anybody to spend time, effort, etc. producing a bio of Rutherford.
Edit: I was just thinking. I guess there are/were some well-known JWs, too (Prince, Michael Jackson, etc.), but maybe the public wasn't aware that they were JWs? I don't know. I still just think that Mormondom is more mainstream because of such people as Mitt Romney and because of the university (Brigham Young University).