Because sex is such a powerful drive it terrifies a lot of people, and so there’s a desire for the “normative narrative” that spells out, like a kind of sexual Bible, what is “normal” and what is not.
Unfortunately of course sexuality is a very complex thing indeed, and when we look at it with inbuilt assumptions and prejudices we won’t see much of what is really happening. It’s good that researchers are beginning to look at more aspects of human sexuality and I hope that experimental design and data analysis are brought up to par (much research isn’t worth the paper it’s ultimately printed on because of flawed design and/or inappropriate statistical techniques used during the data analysis phase). I suspect, however, we will continue to see social attitudes impacting such studies as it is very difficult for people to recognize their own biases. To add to this obvious problem is the fact that researchers often appear quite naive with regards to the variety of sexual activities out in the real world — though this may be partially the result of the fact you likely won’t get a grant to study some of the less “normative” desires and behaviors.