From a Eurocopter training manual, the effect of the strake on the B2 variants is described as follows:
"In sideways flight to the left, the main rotor downwash is deflected and accelerated over the RH side of the tail boom, which induces a negative pressure of approximately 1 mbar/cm2 along the entire tail boom. This reduces the effect of the tail rotor by roughly 5%. A strake added at 45° causes the main rotor downwash flow to separate and restores the pressure to the static value. The effect of the strake is thus to regain the 5% moment and to improve the tail rotor efficiency (including in hover)."
However, a feature found on B3's but not B2's is the added 'step' on the upper vertical fin. The training manual describes the effect of this as follows:
"In cruise flight, the asymmetric NACA airfoil of the dorsal fin generates an aerodynamic force F1 that opposes the main rotor's counter torque CR and thus reinforces the tail rotor thrust. This allows the tail rotor pitch to be reduced and power to be saved. In the B3 version, a step on the RH side of the upper fin improve this phenomenon."
My guess would be that the 'step' on the vertical fin leads to more desirable handling of the machine than the strake (in the manufacturer's view), and there may have been some unwanted aerodynamic effects of having both the strake and the step installed at the same time, so in the evolution of the type, Eurocopter elected to keep the step on the vertical fin and do without the strake.
All just speculation though so I'm happy to be corrected.