Misrepresented PhD program
Sounds like there was a conflict between the two behavioral researchers that left and the rest of the department about the direction of the department. The behavioral researchers lost. Such conflicts can be very nasty. The fact that the conflict was "resolved" by the departure of two faculty, points in that direction. In that case all remaining faculty will be very relieved that that is over, and the department is not going to start that all over again to accommodate a PhD student. In that case, you can either become a non behavioral researcher or switch to another department/university. It is unfair, but if my description of the situation is correct then those are the only realistic options. 

You can ask the researchers that left if they know a department that fits your interests. Switching departments for this reason is perfectly acceptable, this is not your fault.