I think the answer is simpler: it's because you're likely working twice as hard as people exclusively in either field. If you know both fields only superficiall

I think the answer is simpler: it's because you're likely working twice as hard as people exclusively in either field. If you know both fields only superficially, you can't make much of a dent in either. People successful at crossing boundaries simply have done roughly twice as much legwork. As I was once told: "the problem with being a tenured prof in a multidisciplinary department is you have to do tenure-worth work for both sides"