Almost all studies about vegetarianism are born of an agenda to promote vegetarianism.

To a meat eater, studying the effects of eating an all vegetable diet is the same as studying the effects of walking backwards everywhere you go: it's not something we'd ever consider doing so we see no value in studying the effects of doing it. Furthermore, meat eaters do not care if other people don't want to eat meat. As such, most of the people conducting studies about vegetarianism are people who think eating meat is immoral. Their motivation to conduct the study is to stop other people from eating meat.