It seems they tested several fuzes, but only timer fuzes went into mass production. The timer was set before take off in one of three settings, each corresponding to a range.
The main purpose was not to kill buffs, but to break buff formations before the main gun attack, making these formation far more vulnerable and the gun attack much safer.
Using impact fuzes would be very risky when fighting over friendly territory as most of the rockets fired would miss and explode after hitting the ground.