He omits Niven’s Law of Time Travel, though: “If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.” The reason is that if it’s possible to change the past — and, thus the present — change will continue until it reaches the stable state in which no time machine is invented anywhere. Or as Hans Moravec glosses it:

There is a spookier possibility. Suppose it is easy to send messages to the past, but that forward causality also holds (i.e. past events determine the future). In one way of reasoning about it, a message sent to the past will “alter” the entire history following its receipt, including the event that sent it, and thus the message itself. Thus altered, the message will change the past in a different way, and so on, until some “equilibrium” is reached–the simplest being the situation where no message at all is sent. Time travel may thus act to erase itself

So maybe it already has.