It's been said "do not speak ill of the dead". Why? If/only if what's said is TRUE why shouldn't we speak the truth of those living or dead?

Respect both for the dead and their grieving family. And of course the dead person cannot defend themselves against whatever is being said.

Time and place. It’s inappropriate to speak insultingly to those grieving, no matter the truth.
It is however, entirely appropriate in truthful discussions of past history to speak ill of those who clearly did ill. To live otherwise is delusional. 

For some people "speaking the truth" only means judging others and excoriating and condemning them. For what purpose?  Only to make oneself feel better and superior about their own drab life!  Much better I think to accept people and try to creatively get on with them to work together for a better world.