Eliminate false hope
When you leave someone with a vaguely dishonest excuse as to why you're ending the relationship, you not only fail to give them any sense of closure (if such a thing is even possible), you also set yourself up for uncomfortable déjà vu scenarios down the line where you're forced to rehash your reasoning all over again.
Hell, this can even happen when you are clear (consider that stereotypical ex who, after any run-in or minor communication, assumes that a reunion is within reach). The point is, people tend to ruminate over the breakup conversation, searching for hope even when things are as obviously bleak as a Kubrick script. Thus, it is your job to eliminate false hope. Don't say things might change "in the future" or that you "need time" when you know there is no future and that the clock has run out.
It might be initially easier to cloak the conversation in ambiguities, but it's kinder (and more efficient) to just...