I read Adam Kirch's review 'Why Jewish History Is So Hard to Write'. Perhaps it is because no one can ever seem to agree on what 'Jewish' actually is, as evidenced by some of the discussion threads here...
Anecdote: I was once in a conversation with a fellow who was a rabbi and this topic came up. He said, "well...the way I think of it is, if you can think of ten characteristics that are 'Jewish', anyone who has any six of them can claim the title." I guess that's one way to be flexible.