I don't know: There may good reasons against this practice (of refraining from saying anything adverse against the deceased), but there are at least two obvious reasons for this practice:
1. The deceased is unable to defend himself; it is inherently unfair to attack an opponent who is unable to defend himself.
2. Subconscious superstitious belief that the deceased could somehow "avenge" himself.
Regards,
i don’t believe in curses!!! the truth’s the truth.