Disproportionate Force
It would take a much smaller force in general to move an asteroid away from impacting Earth than to use one to target a particular spot. To make a potential impactor miss you simply move it in the easiest miss direction. To target a particular spot would require a massive orbit change unless the impactor were already on course for Earth and even then the energy needed would most likely be more because of less freedom of orbit choices.
Regards,
Mark