Well, basically, two potential reasons:
The first being because Behemoths may not canonically exist. All modern FEV creature variants were created by The Master, using FEV from West-Tek/Mariposa.
Unfortunately that was immediately pre-war, there is no canonical way for FEV to end up in a vault in Maryland that was also pre-war.
The second option preserves the canonicity of FO3 (which may or may not be questionable). In that case it's simply because FEV is amazingly genetically unstable, unique conditions in the east-coast vault lead to it developing the ability to make a behemoth, but for some reason not a floater or a "true" centaur.