I'm curious. Have you actually showed them an example so they can see what it looks like? In my experience working with developers, showing them working code is far more powerful than trying to explain the benefits to them.
I wrote some examples using the Spock framework and, even though it was in Groovy, the Java devs understood the intent and got what I was saying about BDD. Also, let them know how long it took to do the work so they can judge the value of the time spent. If you haven't tried it before, I'd recommend doing a couple of tests because the first one always takes longer - that way you can give them a more real-world expectation. "The first one took a couple of hours because I was having to look everything up; but the second one only took two minutes."
Hoping this helps,
SCJP, SCJD, SCEA 5 "Any sufficiently analyzed magic is indistinguishable from science!" Agatha Heterodyne (Girl Genius)