Firstly, you're right, a dead person cannot speak for themselves and thus it becomes considerably more unfair to defame them than it would be if they had the opportunity to respond. This is why in most countries a dead person cannot be held liable for a criminal offence, for example, because they are not in a position to defend themselves.
Secondly, because human memories are very malleable. It's very easy to put a very different spin on the memories people have of someone's life, for example, simply through convincing rhetoric. Thus, if you defame someone's character when they are dead, you might also be robbing people of their own memories of that person.
Of course, it's not taboo to tell the demonstrable truth about the dead, even if the truth is less than flattering, it's only when it becomes an opinion or a malicious action that it becomes taboo.