The condition of the “onen” (a bereaved person whose close relative who has not yet been buried) and the laws are described here.
There it says:
Because of the need to make and conclude the funeral arrangements, the onen is released from the obligations of prayer, and many other specific positive observances, such as reciting the motzi over bread or the grace after meals. He, therefore, cannot be included in a minyan. While he is exempt from performing the positive commandments, he remains part of society and must obey all the negative commands. The onen should not perform commandments from which he is exempted. This would indicate a lack of concern for the deceased, and the Rabbis insisted that there is no virtue in this action.
So when presented with, say, a drink, he cannot say the requisite blessing. Saying the blessing would take at maximum 10 seconds. Any decision about the funeral arrangements could surely wait 10 secs. Would such a short interruption really “indicate a lack of concern for the deceased”?
Why is it then that a bereaved person cannot even make a short blessing over his food?