The system rightly should not react to tossed hand grenades or ordinary 40mm grenades, just as it shouldn't react to rifle rounds or thrown rocks - those grenades have no chance of penetrating the tank's armor anyway.

If soldiers are exposed, then such a system reacting is a greater threat to their lives than a hand grenade; triggering the system is quite likely to harm nearby troops.

The system is also capable of detecting firearm projectiles, but is specifically tuned not to react. Regardless of what calls may come, there won't be a change because the designers and the customers have already considered that possibility and specifically excluded it as not being worth the risk. This is not a case of being too cheap or short-sighted to do something.